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November 29, 2016November 29, 2016 by Fraser Cain

What Are Multiple Star Systems?

When we do finally learn the full truth about our place in the galaxy, and we’re invited to join the Galactic Federation of Planets, I’m sure we’ll always be seen as a quaint backwater world orbiting a boring single star.

The terrifying tentacle monsters from the nightmare tentacle world will gurgle horrifying, but clearly condescending comments about how we’ve only got a single star in the Solar System.

The beings of pure energy will remark how only truly enlightened civilizations can come from systems with at least 6 stars, insulting not only humanity, but also the horrifying tentacle monsters, leading to another galaxy spanning conflict.

Yes, we’ll always be making up for our stellar deficit in the eyes of aliens, or whatever those creepy blobs use for eyes.

What we lack in sophistication, however, we make up in volume. In our Milky Way, fully 2/3rds of star systems only have a single star. The last 1/3rd is made up of multiple star systems.

The Milky Way as seen from Devil's Tower, Wyoming. Image Credit: Wally Pacholka — The Milky Way as seen from Devil’s Tower, Wyoming. Image Credit: Wally Pacholka

We’re taking binary stars, triple star systems, even exotic 7 star systems. When you mix and match different types of stars in various Odd Couple stellar apartments, the results get interesting.

Consider our own Solar System, where the Sun and planets formed together out a cloud of gas and dust. Gravity collected material into the center of the Solar System, becoming the Sun, while the rest of the disk spun up faster and faster. Eventually our star ignited its fusion furnace, blasting out the rest of the stellar nebula.

But different stellar nebulae can lead to the formation of multiple stars instead. What you get depends on the mass of the cloud, and how fast it’s rotating.

Check out this amazing photograph of a multiple star system forming right now.

ALMA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them. Spiral structure in the dusty disk surrounding them indicates instability in the disk, astronomers said. Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF

In this image, you can see three stars forming together, two at the center, about 60 astronomical units away from each other (60 times the distance from the Earth to the Sun), and then a third orbiting 183 AU away.

It’s estimated these stars are only 10,000 to 20,000 years old. This is one of the most amazing astronomy pictures I ever seen.

When you have two stars, that’s a binary system. If the stars are similar in mass to each other, then they orbit a common point of mass, known as the barycenter. If the stars are different masses, then it can appear that one star is orbiting the other, like a planet going around a star.

When you look up in the sky, many of the single stars you see are actually binary stars, and can be resolved with a pair of binoculars or a small telescope. For example, in a good telescope, Alpha Centauri can be resolved into two equally bright stars, with the much dimmer Proxima Centauri hanging out nearby.

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Skatebiker at English Wikipedia (CC BY-SA 3.0)

You have to be careful, though, sometimes stars just happen to be beside each other in the sky, but they’re not actually orbiting one another – this is known as an optical binary. It’s a trap.

Astronomers find that you can then get binary stars with a third companion orbiting around them. As long as the third star is far enough away, the whole system can be stable. This is a triple star system.

You can get two sets of binary stars orbiting each other, for a quadruple star system.

In fact, you can build up these combinations of stars up. For example, the star system Nu Scorpii has 7 stars in a single system. All happily orbiting one another for eons.

If stars remained unchanging forever, then this would be the end of our story. However, as we’ve discussed in other articles, stars change over time, bloating up as red giants, detonating as supernovae and turning into bizarre objects, like white dwarfs, neutron stars and even black holes. And when these occur in multiple star systems, well, watch the sparks fly.

There are a nearly infinite combinations you can have here: main sequence, red giant, white dwarf, neutron star, and even black holes. I don’t have time to go through all the combinations, but here are some highlights.

This artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either with the formation of a single giant star or as a future binary black hole. ESO/L. Calçada — VFTS 352 is the hottest and most massive double star system to date where the two components are in contact and sharing material. ESO/L. Calçada

For starters, binary stars can get so close they actually touch each other. This is known as a contact binary, where the two stars actually share material back and forth. But it gets even stranger.

When a main sequence star like our Sun runs out of hydrogen fuel in its core, it expands as a red giant, before cooling and becoming a white dwarf.

When a red giant is in a binary system, the distance and evolution of its stellar companion makes all the difference.

If the two stars are close enough, the red giant can pass material over to the other star. And if the red giant is large enough, it can actually engulf its companion. Imagine our Sun, orbiting within the atmosphere of a red giant star. Needless to say, that’s not healthy for any planets.

An even stranger contact binary happens when a red giant consumes a binary neutron star. This is known as a Thorne-Zytkow object. The neutron star spirals inward through the atmosphere of the red giant. When it reaches the core, it either becomes a black hole, gobbling up the red giant from within, or an even more massive neutron star. This is exceedingly rare, and only one candidate object has ever been observed.

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color. Credit: NASA/CXC/M. Weiss — A white dwarf accreting material from a companion star. Credit: NASA/CXC/M. Weiss

When a binary pair is a white dwarf, the dead remnant of a star like our Sun, then material can transfer to the surface of the white dwarf, causing novae explosions. And if enough material is transferred, the white dwarf explodes as a Type 1A supernova.

If you’re a star that was unlucky enough to be born beside a very massive star, you can actually kicked off into space when it explodes as a supernova. In fact, there are rogue stars which such a kick, they’re on an escape trajectory from the entire galaxy, never to return.

If you have two neutron stars in a binary pair, they release energy in the form of gravitational waves, which causes them to lose momentum and spiral inward. Eventually they collide, becoming a black hole, and detonating with so much energy we can see the explosions billions of light-years away – a short-period gamma ray burst.

The combinations are endless.

How Earth could look with two suns. Credit: NASA/JPL-Caltech/Univ. of Ariz.

It’s amazing to think what the night sky would look like if we were born into a multiple star system. Sometimes there would be several stars in the sky, other times just one. And rarely, there would be an actual night.

How would life be different in a multiple star system? Let me know your thoughts in the comments.

In our next episode, we try to untangle this bizarre paradox. If the Universe is infinite, how did it start out as a singularity? That doesn’t make any sense.

We glossed over it in this episode, but one of the most interesting effects of multiple star systems are novae, explosions of stolen material on the surface of a white dwarf star. Learn more about it in this video.

October 20, 2016 by Fraser Cain

What is a Nova?

There are times when I really wish astronomers could take their advanced modern knowledge of the cosmos and then go back and rewrite all the terminology so that they make more sense. For example, dark matter and dark energy seem like they’re linked, and maybe they are, but really, they’re just mysteries.

Is dark matter actually matter, or just a different way that gravity works over long distances? Is dark energy really energy, or is it part of the expansion of space itself. Black holes are neither black, nor holes, but that doesn’t stop people from imagining them as dark tunnels to another Universe. Or the Big Bang, which makes you think of an explosion.

Another category that could really use a re-organizing is the term nova, and all the related objects that share that term: nova, supernova, hypernova, meganova, ultranova. Okay, I made those last couple up.

I guess if you go back to the basics, a nova is a star that momentarily brightens up. And a supernova is a star that momentarily brightens up… to death. But the underlying scenario is totally different.

New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these "time bombs" could be scattered throughout our Galaxy. In this artist's conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA) — In this artist’s conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA)

As we’ve mentioned in many articles already, a supernova commonly occurs when a massive star runs out of fuel in its core, implodes, and then detonates with an enormous explosion. There’s another kind of supernova, but we’ll get to that later.

A plain old regular nova, on the other hand, happens when a white dwarf – the dead remnant of a Sun-like star – absorbs a little too much material from a binary companion. This borrowed hydrogen undergoes fusion, which causes it to brighten up significantly, pumping up to 100,000 times more energy off into space.

Imagine a situation where you’ve got two main sequence stars like our Sun orbiting one another in a tight binary system. Over the course of billions of years, one of the stars runs out of fuel in its core, expands as a red giant, and then contracts back down into a white dwarf. It’s dead.

Some time later, the second star dies, and it expands as a red giant. So now you’ve got a red dwarf and a white dwarf in this binary system, orbiting around and around each other, and material is streaming off the red giant and onto the smaller white dwarf.

Illustration of a white dwarf feeding off its companion star Credit: ESO / M. Kornmesser

This material piles up on the surface of the white dwarf forming a cosy blanket of stolen hydrogen. When the surface temperature reaches 20 million kelvin, the hydrogen begins to fuse, as if it was the core of a star. Metaphorically speaking, its skin catches fire. No, wait, even better. Its skin catches fire and then blasts off into space.

Over the course of a few months, the star brightens significantly in the sky. Sometimes a star that required a telescope before suddenly becomes visible with the unaided eye. And then it slowly fades again, back to its original brightness.

Some stars do this on a regular basis, brightening a few times a century. Others must clearly be on a longer cycle, we’ve only seen them do it once.

Astronomers think there are about 40 novae a year across the Milky Way, and we often see them in other galaxies.

The term “nova” was first coined by the Danish astronomer Tycho Brahe in 1572, when he observed a supernova with his telescope. He called it the “nova stella”, or new star, and the name stuck. Other astronomers used the term to describe any star that brightened up in the sky, before they even really understood the causes.

During a nova event, only about 5% of the material gathered on the white dwarf is actually consumed in the flash of fusion. Some is blasted off into space, and some of the byproducts of fusion pile up on its surface.

Tycho's Supernova Remnant. Credit: Spitzer, Chandra and Calar Alto Telescopes. — Tycho’s Supernova Remnant. Credit: Spitzer, Chandra and Calar Alto Telescopes.

Over millions of years, the white dwarf can collect enough material that carbon fusion can occur. At 1.4 times the mass of the Sun, a runaway fusion reaction overtakes the entire white dwarf star, releasing enough energy to detonate it in a matter of seconds.

If a regular nova is a quick flare-up of fusion on the surface of a white dwarf star, then this event is a super nova, where the entire star explodes from a runaway fusion reaction.

You might have guessed, this is known as a Type 1a supernova, and astronomers use these explosions as a way to measure distance in the Universe, because they always explode with the same amount of energy.

Hmm, I guess the terminology isn’t so bad after all: nova is a flare up, and a supernova is a catastrophic flare up to death… that works.

Now you know. A nova occurs when a dead star steals material from a binary companion, and undergoes a momentary return to the good old days of fusion. A Type Ia supernova is that final explosion when a white dwarf has gathered its last meal.

July 8, 2016February 22, 2017 by Fraser Cain

What if Earth Stopped Orbiting the Sun?

In a previous article I investigated what would happen if the Earth stopped turning entirely, either locking to the Sun or the background stars.

If it happened quickly, then results would be catastrophic, turning the whole planet into a blended slurry of mountains, oceans and trees, hurting past a hundreds of kilometers per hour. And if it happened slowly, it would still be unpleasant, as we stopped having a proper day/night cycle. But it wouldn’t be immediately lethal.

But would happen if the Earth somehow just stopped in its tracks as it was orbiting the Sun, as if it ran into an invisible wall? As with the Earth turning question, it’s completely and totally impossible; it’s not going to happen. And with the unspun Earth, it would be totally devastating and super interesting to imagine.

A view of Earth on October 24, 2014 from the Chinese Chang’e-5 T1 spacecraft. Credit: Xinhua News, via UnmannedSpaceflight.com. — Credit: Xinhua News, via UnmannedSpaceflight.com.

Before we begin to imagine the horrifying consequences of a total loss of orbital velocity, let’s examine the physics involved.

The Earth is traveling around the Sun with an orbital velocity of 30 kilometers per second. This is exactly the speed it needs to be going to counteract the force of gravity from the Sun pulling it inward. If the Sun were to suddenly disappear, Earth would travel in a perfectly straight line at 30 km/s. This is how orbits work.

If the Earth’s orbital velocity sped up, then it would go into a higher orbit to compensate. And if the Earth’s orbital velocity slowed down, then it fall into a lower orbit to compensate. And if the Earth’s orbital velocity was slowed all the way down to zero? Now we’re cooking, literally.

First, let’s imagine what would happen if the Earth just suddenly stopped.

As I mentioned above, the Earth’s orbital velocity is 30 km/s, which means that if it suddenly stopped, everything on it would still have 30 km/s worth of inertia. The escape velocity of the Earth is about 11 km/s.

In other words, anything on the Earth’s leading side would fly off into space, continuing along the Earth’s orbital path around the Sun. Anything on the trailing side would be pulverized against the Earth. It would be a horrible, gooey mess.

But even if the Earth slowed gently to a stop, it would still be a horrible mess. Without the outward centripetal force to counteract the inward pull of gravity, the Earth would begin falling towards the Sun.

How long would it take? My integral calculus is a little rusty, so I’ll draw upon the calculations of Dave Rothstein from Cornell’s Ask an Astronomer. According to Dr. Rothstein, the whole journey would take about 65 days. It would take 41 days to cross the orbit of Venus, and on day 57, we’d cross the orbit of Mercury.

As they days went by, the Earth would get hotter and hotter as it got closer to the Sun. Aatish Bhatia over at WIRED did some further calculations to figure out the temperature. A month into the freefall, and the average temperature on Earth would have risen to 50 degrees C. 50 days in and we’d be about 125 C. On the final day, we’d get up to 3,000 C… and then, that would be that.

Of course, this is completely and totally impossible. There’s no force that could just stop the Earth in its tracks like that. There is, however, a plausible scenario that might drag the Earth into the Sun.

In the far future, the Sun will turn into a red giant and expand outward, engulfing the orbits of Mercury and Venus. There’s still an argument among astronomers on whether it’s going to gobble up Earth as well.

Illustration of the red supergiant Betelgeuse, as seen from a fictional orbiting world. © Digital Drew. — Poor Earth. © Digital Drew.

Let’s say it does. In that case, the Earth will be inside the atmosphere of the Sun, and experience a friction from the solar material as it orbits around, and spiral inward. Of course, at this point you’re orbiting inside the Sun, so falling into the Sun already happened.

There you go. If the Earth happened to stop dead in its orbit, it would take about 65 days to plunge down into the Sun, disappearing in a puff of plasma.

May 12, 2016October 16, 2016 by Fraser Cain

What is the Biggest Star in the Universe?

This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!

My six-year old daughter is a question-asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “and there’s no way we can possibly know what the biggest star is”. But that’s not a real answer.

So she refined the question. “What’s the biggest star that we know of?” Of course, I was stuck in the car, and without access to the Internet. But once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?

Solar Radius and Mass:

When talking about the size of stars, it’s important to first take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 10³⁰ kilograms (4.3 x 10³⁰ pounds). That’s 2 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.

Artist's depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons — *Artist’s depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons*

Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.

Classification:

To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).

In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.

The Hertzspirg-Russel diagram, showing the relation between star's color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com — *The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com*

There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).

On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.

Eta Carinae, one of the most massive stars known. Image credit: NASA — *Eta Carinae, one of the most massive stars known, located in the Carina constellation. Credit: NASA*

There is some controversy over this world’s size though. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.

In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!

Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.

But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.

The Largest Star: UY Scuti

As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.

However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.

*A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone*

Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.

And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.

These are the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them. But even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.

VY Canis Majoris. The biggest known star. — *Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen*

As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).

But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!

And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!

We have written many articles about stars for Universe Today. Here’s The Sun, What’s the Brightest Star in the Sky Past and Future?, and What Is The Smallest Star?

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.

December 22, 2015September 24, 2016 by Matt Williams

What is the Life Cycle Of The Sun?

The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser

The Sun has always been the center of our cosmological systems. But with the advent of modern astronomy, humans have become aware of the fact that the Sun is merely one of countless stars in our Universe. In essence, it is a perfectly normal example of a G-type main-sequence star (G2V, aka. “yellow dwarf”). And like all stars, it has a lifespan, characterized by a formation, main sequence, and eventual death.

This lifespan began roughly 4.6 billion years ago, and will continue for about another 4.5 – 5.5 billion years, when it will deplete its supply of hydrogen, helium, and collapse into a white dwarf. But this is just the abridged version of the Sun’s lifespan. As always, God (or the Devil, depending on who you ask) is in the details!

To break it down, the Sun is about half way through the most stable part of its life. Over the course of the past four billion years, during which time planet Earth and the entire Solar System was born, it has remained relatively unchanged. This will stay the case for another four billion years, at which point, it will have exhausted its supply of hydrogen fuel. When that happens, some pretty drastic things will take place!

The Birth of the Sun:

According to Nebular Theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

Protoplanet Hypothesis — *Artist’s concept of a star surrounded by a molecular cloud to form a swirling disk called a “protoplanetary disk.” Credit: NASA/JPL-Caltech*

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it.

The ball at the center would eventually form the Sun, while the disk of material would form the planets. The Sun spent about 100,000 years as a collapsing protostar before temperature and pressures in the interior ignited fusion at its core. The Sun started as a T Tauri star – a wildly active star that blasted out an intense solar wind. And just a few million years later, it settled down into its current form. The life cycle of the Sun had begun.

The Main Sequence:

The Sun, like most stars in the Universe, is on the main sequence stage of its life, during which nuclear fusion reactions in its core fuse hydrogen into helium. Every second, 600 million tons of matter are converted into neutrinos, solar radiation, and roughly 4 x 10²⁷ Watts of energy. For the Sun, this process began 4.57 billion years ago, and it has been generating energy this way every since.

However, this process cannot last forever since there is a finite amount of hydrogen in the core of the Sun. So far, the Sun has converted an estimated 100 times the mass of the Earth into helium and solar energy. As more hydrogen is converted into helium, the core continues to shrink, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force.

The Sun captured by NASA's Solar Dynamics Observatory Spacecraft. — *The Sun captured by NASA’s Solar Dynamics Observatory Spacecraft.*

This places more pressure on the core, which is resisted by a resulting increase in the rate at which fusion occurs. Basically, this means that as the Sun continues to expend hydrogen in its core, the fusion process speeds up and the output of the Sun increases. At present, this is leading to a 1% increase in luminosity every 100 million years, and a 30% increase over the course of the last 4.5 billion years.

In 1.1 billion years from now, the Sun will be 10% brighter than it is today, and this increase in luminosity will also mean an increase in heat energy, which Earth’s atmosphere will absorb. This will trigger a moist greenhouse effect here on Earth that is similar to the runaway warming that turned Venus into the hellish environment we see there today.

In 3.5 billion years from now, the Sun will be 40% brighter than it is right now. This increase will cause the oceans to boil, the ice caps to permanently melt, and all water vapor in the atmosphere to be lost to space. Under these conditions, life as we know it will be unable to survive anywhere on the surface. In short, planet Earth will come to be another hot, dry Venus.

Core Hydrogen Exhaustion:

All things must end. That is true for us, that is true for the Earth, and that is true for the Sun. It’s not going to happen anytime soon, but one day in the distant future, the Sun will run out of hydrogen fuel and slowly slouch towards death. This will begin in approximate 5.4 billion years, at which point the Sun will exit the main sequence of its lifespan.

With its hydrogen exhausted in the core, the inert helium ash that has built up there will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size and enter the Red Giant phase of its evolution. It is calculated that the expanding Sun will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth. Even if the Earth survives, the intense heat from the red sun will scorch our planet and make it completely impossible for life to survive.

Final Phase and Death:

Once it reaches the Red-Giant-Branch (RGB) phase, the Sun will haves approximately 120 million years of active life left. But much will happen in this amount of time. First, the core (full of degenerate helium), will ignite violently in a helium flash – where approximately 6% of the core and 40% of the Sun’s mass will be converted into carbon within a matter of minutes.

The Sun will then shrink to around 10 times its current size and 50 times its luminosity, with a temperature a little lower than today. For the next 100 million years, it will continue to burn helium in its core until it is exhausted. By this point, it will be in its Asymptotic-Giant-Branch (AGB) phase, where it will expand again (much faster this time) and become more luminous.

Over the course of the next 20 million years, the Sun will then become unstable and begin losing mass through a series of thermal pulses. These will occur every 100,000 years or so, becoming larger each time and increasing the Sun’s luminosity to 5,000 times its current brightness and its radius to over 1 AU.

At this point, the Sun’s expansion will either encompass the Earth, or leave it entirely inhospitable to life. Planets in the Outer Solar System are likely to change dramatically, as more energy is absorbed from the Sun, causing their water ices to sublimate – perhaps forming dense atmosphere and surface oceans. After 500,000 years or so, only half of the Sun’s current mass will remain and its outer envelope will begin to form a planetary nebula.

The post-AGB evolution will be even faster, as the ejected mass becomes ionized to form a planetary nebula and the exposed core reaches 30,000 K. The final, naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black.

Ultimate Fate of our Sun:

When people think of stars dying, what typically comes to mind are massive supernovas and the creation of black holes. However, this will not be the case with our Sun, due to the simple fact that it is not nearly massive enough. While it might seem huge to us, but the Sun is a relatively low mass star compared to some of the enormous high mass stars out there in the Universe.

As such, when our Sun runs out of hydrogen fuel, it will expand to become a red giant, puff off its outer layers, and then settle down as a compact white dwarf star, then slowly cooling down for trillions of years. If, however, the Sun had about 10 times its current mass, the final phase of its lifespan would be significantly more (ahem) explosive.

When this super-massive Sun ran out of hydrogen fuel in its core, it would switch over to converting atoms of helium, and then atoms of carbon (just like our own). This process would continue, with the Sun consuming heavier and heavier fuel in concentric layers. Each layer would take less time than the last, all the way up to nickel – which could take just a day to burn through.

Then, iron would starts to build up in the core of the star. Since iron doesn’t give off any energy when it undergoes nuclear fusion, the star would have no more outward pressure in its core to prevent it from collapsing inward. When about 1.38 times the mass of the Sun is iron collected at the core, it would catastrophically implode, releasing an enormous amount of energy.

Within eight minutes, the amount of time it takes for light to travel from the Sun to Earth, an incomprehensible amount of energy would sweep past the Earth and destroy everything in the Solar System. The energy released from this might be enough to briefly outshine the galaxy, and a new nebula (like the Crab Nebula) would be visible from nearby star systems, expanding outward for thousands of years.

All that would remain of the Sun would be a rapidly spinning neutron star, or maybe even a stellar black hole. But of course, this is not to be our Sun’s fate. Given its mass, it will eventually collapse into a white star until it burns itself out. And of course, this won’t be happening for another 6 billion years or so. By that point, humanity will either be long dead or have moved on. In the meantime, we have plenty of days of sunshine to look forward to!

We have written many interesting articles on the Sun here at Universe Today. Here’s What Color Is The Sun?, What Kind of Star is the Sun?, How Does The Sun Produce Energy?, and Could We Terraform the Sun?

Astronomy Cast also has some interesting episodes on the subject. Check them out- Episode 30: The Sun, Spots and All, Episode 108: The Life of the Sun, Episode 238: Solar Activity.

For more information, check out NASA’s Solar System Guide.

October 15, 2015February 23, 2017 by Fraser Cain

Why Do Red Giants Expand?

We know that the Sun will last another 5 billion years and then expand us a red giant. What will actually make this process happen?

One of the handy things about the Universe, apart from the fact that it exists, is that it lets us see crazy different configurations of everything, including planets, stars and galaxies.

We see stars like our Sun and dramatically unlike our Sun. Tiny, cool red dwarf stars with a fraction of the mass of our own, sipping away at their hydrogen juice boxes for billions and even trillions of years. Stars with way more mass than our own, blasting out enormous amounts of radiation, only lasting a few million years before they detonate as supernovae.

There are ones younger than the Sun; just now clearing out the gas and dust in their solar nebula with intense ultraviolet radiation. Stars much older than ours, bloated up into enormous sizes, nearing the end of their lives before they fade into their golden years as white dwarfs.

The Sun is a main sequence star, converting hydrogen into helium at its core, like it’s been doing for more than 4.5 billion years, and will continue to do so for another 5 or so. At the end of its life, it’s going to bloat up as a red giant, so large that it consumes Mercury and Venus, and maybe even Earth.

What’s the process going on inside the Sun that makes this happen? Let’s peel away the Sun and take a look at the core. After we’re done screaming about the burning burning hands, we’ll see that the Sun is this enormous sphere of hydrogen and helium, 1.4 million kilometers across, the actual business of fusion is happening down in the core, a region that’s a delicious bubblegum center a tiny 280,000 kilometers across.

The core is less than one percent of the entire volume, but because the density of hydrogen in the chewy center is 150 times more than liquid water, it accounts for a freakishly huge 35% of its mass.

It’s thanks to the mass of the entire star, 2 x 10^30 kg, bearing down on the core thanks to gravity. Down here in the core, temperatures are more than 15 million degrees Celsius. It’s the perfect spot for nuclear fusion picnic.

There are a few paths fusion can take, but the main one is where hydrogen atoms are mushed into helium. This process releases enough gamma radiation to make you a planet full of Hulks.

Proton-proton fusion in a sun-like star. Credit: Borb

While the Sun has been performing hydrogen fusion, all this helium has been piling up at its core, like nuclear waste. Terrifyingly, it’s still fuel, but our little Sun just doesn’t have the temperature or pressure at its core to be able to use it.

Eventually, the fusion at the core of the Sun shuts down, choked off by all this helium and in a last gasp of high pitched mickey mouse voice terror the helium core begins to contract and heat up. At this point, an amazing thing happens. It’s now hot enough for a layer of hydrogen just around the core to heat up and begin fusion again. The Sun now gets a second chance at life.

As this outer layer contains a bigger volume than the original core of the Sun, it heats up significantly, releasing far more energy. This increase in light pressure from the core pushes much harder against gravity, and expands the volume of the Sun.

Even this isn’t the end of the star’s life. Dammit, Harkness, just stay down. Helium continues to build up, and even this extra shell around the core isn’t hot and dense enough to support fusion. So the core dies again. The star begins to contract, the gravitational energy heats up again, allowing another shell of hydrogen to have the pressure and temperature for fusion, and then we’re back in business!

Our Sun will likely go through this process multiple times, each phase taking a few years to complete as it expands and contracts, heats and cools. Our Sun becomes a variable star.

Eventually, we run out of usable hydrogen, but fortunately, it’s able to switch over to using helium as fuel, generating carbon and oxygen as byproducts. This doesn’t last long, and when it’s gone, the Sun gets swollen to hundreds of times its size, releasing thousands of times more energy.

This is when the Sun becomes that familiar red giant, gobbling up the tasty planets, including, quite possibly the Earth.The remaining atmosphere puffs out from the Sun, and drifts off into space creating a beautiful planetary nebula that future alien astronomers will enjoy for thousands of years. What’s left is a carbon oxygen core, a white dwarf.

The Sun is completely out of tricks to make fusion happen any more, and it’ll now cool down to the background temperature of the Universe. Our Sun will die in a dramatic way, billions of years from now when it bloats up 500 times its original volume.

What do you think future alien astronomers will call the planetary nebula left behind by the Sun? Give it a name in the comments below.

March 13, 2014March 8, 2017 by Fraser Cain

How Do You Jumpstart A Dead Star?

It’s a staple of science fiction, restarting our dying star with some kind of atomic superbomb. Why is our Sun running out of fuel, and what can we actually do to get it restarted?

Stars die. Occasionally threatening the Earth and its civilization in a variety plot devices in science fiction. Fortunately there’s often a Bruce Willis coming in to save the day, delivering a contraption, possibly riding a giant bomb shaped like a spaceship, to the outer proximity of our dying Sun that magically fixes the broken star and all humanity is saved.

Is there any truth in this idea? If our Sun dies, can we just crack out a giant solar defibrillator and shock it back into life? Not exactly.

First, let’s review at how stars die. Our Sun is halfway through its life. It’s been going for about 4.5 billion years, and in 5 billion years it’ll use up all the hydrogen in its core, bloat up as a red giant, puff off its outer layers and collapse down into a white dwarf.

Is there a point in there, anywhere, that we could get it back to acting like a sun? Technically? Yes. Did you know it will only use up a fraction of its fuel during its lifetime? Only in the core of the Sun are the temperatures and pressures high enough for fusion reactions to take place. This region extends out to roughly 25% of the radius, which only makes up about 2% of the volume.

Outside the core is the radiative zone, where fusion doesn’t take place. Here, the only way gamma radiation can escape is to be absorbed and radiated countless times, until it reaches the next layer of the Sun: the convective zone. Here temperatures have dropped to the point that the whole region acts like a giant lava lamp. Huge blobs of superheated stellar plasma rise up within the star and release their energy into space. This radiative zone acts like a wall, keeping the potential fuel in the convective zone away from the fusion furnace.

Cutaway to the Interior of the Sun. Credit: NASA

So, if you could connect the convective zone to the solar core, you’d be able to keep mixing up the material in the Sun. The core of the Sun would be able to efficiently fuse all the hydrogen in the star.

Sound crazy? Interestingly, this already happens in our Universe. For red dwarf stars with less than 35% the mass of the Sun, their convective zones connect directly to the core of the star. This is why these stars can last for hundreds of billions and even trillions of years. They will efficiently use up all the hydrogen in the entire star thanks to the mixing of the convective zone. If we could create a method to break through the radiative zone and get that fresh hydrogen into the core of the Sun, we could keep basking in its golden tanning rays for well past its current expiration date.

I never said it would be easy. It would take stellar engineering at a colossal scale to overcome the equilibrium of the star. A future civilization with an incomprehensible amount of energy and stellar engineering ability might be able to convert our one star into a collection of fully convective red dwarf stars. And these could sip away their hydrogen for trillions of years.

Tell us in the comments on how you think we should go about it. My money is on giant ‘magic bullet’ blender” or a perhaps a Dyson solar juicer.

January 29, 2014March 8, 2017 by Fraser Cain

What Is The Future Of Our Sun?

Who knows what the future holds for our Sun? Dr. Mark Morris, a professor of astronomy at UCLA sure knows. Professor Morris sat down with us to let us know what we’re in for over the next few billions years.

“Hi, I’m Professor Mark Morris. I’m teaching at UCLA where I also carry out my research. I work on the center of the galaxy and what’s going on there – in this fabulous arena there, and on dying stars – stars that have reached the end of their lifetime and are putting on a display for us as they do so.”

What is the future of our sun?

“Well, there’s every expectation that in about 5 billion more years, that our sun will swell up to become a red giant. And then, as it gets larger and larger, it will eventually become what’s called an asymptotic giant branch star – a star whose radius is just under the distance between the sun and the Earth – one astronomical unit in size. So the Earth will be literally skimming the surface of the red giant sun when it’s an asymptotic giant branch star.”

“A star that big is also cool because they’re cold – red hot versus blue hot or yellow hot like our sun. Because it’s cold, a red giant star at its surface layers can keep all of its elements in the gas phase. So some of the heavier elements – the metals and the silicates – condense out as small dust grains, and when these elements condense out as solids, then radiation pressure from this very luminous giant star pushes the dust grains out. That may seem like a minor issue, but in fact these dust grains carry the gas with them. And so the star literally expels its atmosphere, and goes from a red giant star to a white dwarf, when finally the core of the star is exposed. Now, as it’s doing this, that hot core of the star is still very luminous and lights up through a fluorescent process, this out-flowing envelope, this atmosphere that was once a star, and that’s what produces these beautiful displays that are called planetary nebulae.”

“Now, planetary nebulae can be these beautiful round, spherical objects, or they can be bipolar, which is one of the mysteries that we’re working here is trying to understand why, at some stage, a star suddenly becomes axisymmetric – in other words, is sending out is’s atmosphere in two diametrically opposed directions predominantly, rather than continuing to lose mass spherically.”

“We can’t invoke rotation of the star – that would be one way to get a preferred axis, but stars don’t rotate fast enough. If you take the sun and let it expand to become a red giant, then by the conservation of angular momentum, it literally won’t be spinning at all. It’ll be spinning so slowly that it’ll literally have no effect. So we can’t invoke spin, so there must be something going on deep down inside the star, that when you finally expose some rapidly spinning core, it can have an effect.”

“Or, all of the stars that we see as planetary nebula can have binary companions, that could be massive planets or relatively low mass stars that themselves can impose an angular momentum orientation on the system. This is in fact an idea that I’ve been championing for decades now, and it has some traction. There’s a lot of planetary nebula nuclei, the white dwarves, that seem to have companions near them that are suspect for having been responsible for helping strip the atmosphere of the mass-losing red giant star but also providing a preferred axis along which the ejected matter can flow.”

January 6, 2014March 14, 2017 by Fraser Cain

Will The Sun Explode?

All stars die, some more violently than others.

Once our own Sun has consumed all the hydrogen fuel in its core, it too will reach the end of its life. Astronomers estimate this to be a short 7 billion years from now. For a few million years, it will expand into a red giant, puffing away its outer layers. Then it’ll collapse down into a white dwarf and slowly cool down to the background temperature of the Universe.

I’m sure you know that some other stars explode when they die. They also run out of fuel in their core, but instead of becoming a red giant, they detonate in a fraction of a second as a supernova.

So, what’s the big difference between stars like our Sun and the stars that can explode as supernovae?

Mass. That’s it.

Supernova progenitors – these stars capable of becoming supernovae – are extremely massive, at least 8 to 12 times the mass of our Sun. When a star this big runs out of fuel, its core collapses. In a fraction of a second, material falls inward to creating an extremely dense neutron star or even a black hole. This process releases an enormous amount of energy, which we see as a supernova.

If a star has even more mass, beyond 140 times the mass of the Sun, it explodes completely and nothing remains at all. If these other stars can detonate like this, is it possible for our Sun to explode?

Could there be some chain reaction we could set off, some exotic element a rare comet could introduce on impact, or a science fiction doomsday ray we could fire up to make the Sun explode?

Nope, quite simply, it just doesn’t have enough mass. The only way this could ever happen is if it was much, much more massive, bringing it to that lower supernovae limit.

In other words, you would need to crash an equally massive star into our Sun. And then do it again, and again.. and again… another half dozen more times. Then, and only then would you have an object massive enough to detonate as a supernova.

We don't have to worry about our sun exploding into a supernova. — We don’t have to worry about our sun exploding into a supernova.

Now, I’m sure you’re all resting easy knowing that solar detonation is near the bottom of the planetary annihilation list. I’ve got even better news. Not only will this never happen to the Sun, but there are no large stars close enough to cause us any damage if they did explode.
A supernova would need to go off within a distance of 100 light-years to irradiate our planet.

According to Dr. Phil Plait from Bad Astronomy, the closest star that could detonate as a supernova is the 10 solar mass Spica, at a distance of 260 light-years. No where near close enough to cause us any danger.

So don’t worry about our Sun exploding or another nearby star going supernova and wiping us out. You can put your feet up and relax, as it’s just not going to happen.

July 6, 2012December 23, 2015 by Jason Major

The Last Outbursts of a Dying Star

As stars approach the inevitable ends of their lives they run out of stellar fuel and begin to lose a gravitational grip on their outermost layers, which can get periodically blown far out into space in enormous gouts of gas — sometimes irregularly-shaped, sometimes in a neat sphere. The latter is the case with the star above, a red giant called U Cam in the constellation Camelopardalis imaged by the Hubble Space Telescope.

From the Hubble image description:

U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds. Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the center of the image. Its brightness, however, is enough to saturate the camera’s receptors, making the star look much bigger than it really is.

The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable, the shell of gas expelled from U Cam is almost perfectly spherical.

Image credit: ESA/NASA