Universe Today

February 11, 2009December 24, 2015 by Ian O'Neill

Stellar Jets are Born Knotted

Herbig Haro object HH47 (a stellar jet), observed with the Hubble Space Telescope

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Some of the most beautiful structures observed in the Universe are the intricate jets of supersonic material speeding away from accreting stars, such as young proto-stars and stellar mass black holes. These jets are composed of highly collimated gas, rapidly accelerated and ejected from circumstellar accretion disks. The in-falling gas from the disks, usually feeding the black hole or hungry young star, is somehow redirected and blown into the interstellar medium (ISM).

Much work is being done to understand how accretion disk material is turned into a rapid outflow, forming an often knotted, clumpy cloud of outflowing gas. The general idea was that the stellar jet is ejected in a steady flow (like a fire hose), only for it to interact with the surrounding ISM, breaking up as it does so. However, a unique collaboration between plasma physicists, astronomers and computational scientists may have uncovered the true nature behind these knotted structures. They didn’t become knotted, they were born that way…

“The predominant theory says that jets are essentially fire hoses that shoot out matter in a steady stream, and the stream breaks up as it collides with gas and dust in space—but that doesn’t appear to be so after all,” said Adam Frank, professor of astrophysics at the University of Rochester, and co-author of the recent publication. According to Frank, the exciting results uncovered by the international collaboration suggest that far from being a steady stream of gas being ejected from the circumstellar accretion disk, the jets are “fired out more like bullets or buckshot.” It is therefore little wonder that the vast stellar jets appear twisted, knotted and highly structured.

A member of the collaboration, Professor Sergey Lebedev and his team at the Imperial College London, made an attempt to replicate the physics of a star in the laboratory, and the experiment matched the known physics of stellar jets very well. The pioneering work by Lebedev is being lauded a possibly the “best” astrophysical experiment that’s ever been carried out.

Using an aluminium disk, Lebedev applied a high-powered pulse of energy to it. Within the first few billionths of a second, the aluminium began to evaporate, generating a small cloud of plasma. This plasma became an accretion disk analogue, a microscopic equivalent of the plasma being dragged into a proto-star. In the centre of the disk, the aluminium had eroded completely, creating a hole. Through this hole, a magnetic field, being applied below the disk, could penetrate through.

It would appear that the dynamics of the magnetic field interacting with the plasma accurately depicts the observed characteristics of extended stellar jets. At first, the magnetic field pushes the plasma aside around the disk’s hole, but its structure evolves by creating a bubble, then twisting and warping, forming a knot in the plasma jet. Then, a very important event occurs; the initial magnetic “bubble” pinches off and is propelled away. Another magnetic bubble forms to continue the process all over again. These dynamic processes cause packets of plasma to be released in bursts and not in the steady, classical “fire hose” manner.

“We can see these beautiful jets in space, but we have no way to see what the magnetic fields look like,” says Frank. “I can’t go out and stick probes in a star, but here we can get some idea—and it looks like the field is a weird, tangled mess.”

By shrinking this cosmic phenomenon into a laboratory experiment, the investigators have shed some light on the possible mechanism driving the structure of stellar jets. It appears that magnetic processes, not ISM interactions, shape the knotted structure of stellar jets when they born, not after they have evolved.

Source: EurekAlert

February 10, 2009December 24, 2015 by Fraser Cain

What is the Light From Stars?

Star classifications. Image credit: Kieff

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Look into the night sky and you’ll see stars in all directions shining and twinkling in the dark. But what is the light that we’re seeing, and how does it get all the way from the distant stars to here?

All stars are just vast balls of hot plasma. They’re made up of mostly hydrogen and helium, with trace amounts of other elements. Mutual gravity holds the star together, and compresses it inward. Without some kind of force pushing back, stars would just compress themselves down to the size of the Earth, or even smaller.

But as a star gets smaller, the gravitational friction causes it to heat up in its core. When the core of the star reaches about 15 million Kelvin, hydrogen fusion can begin. In this process, atoms of hydrogen are crushed together through a multi-stage process to form helium. This reaction is exothermic, which means that it gives more energy than it gives off. A star like the Sun is releasing 3.86 x 10²⁶ joules of gamma radiation every second.

These photons of energy are trapped inside the star and have to get out. Over a journey that can take more than 100,000 years, the photons are continuously emitted and then absorbed by atoms in the Sun. Each of these jumps can cause the photon to lose energy. When they finally reach the surface of the star, they’ve lost a tremendous amount of energy, and have fallen from high energy gamma rays down to visible wavelengths.

And then, the photons are released from the surface of the star, and free to cross the vacuum of space. Unless they encounter anything, they’ll keep traveling in a straight line for millions, billions and even trillions of years. When you step outside and look at a star that could be a few hundred light-years away, your eyes are the first things the photons have bumped into since they left the surface of the star!

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about how many stars there are in the Milky Way.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.jet.efda.org/fusion-basics/what-is-fusion/
http://www.ips.gov.au/Category/Educational/The%20Sun%20and%20Solar%20Activity/General%20Info/Solar_Constant.pdf

February 10, 2009December 24, 2015 by Fraser Cain

Hypergiant Stars

Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook

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The vast majority of stars out there are tiny red dwarfs, then come the solar mass stars like our Sun. There are giant stars and even supergiant stars. But the biggest stars out there are the monstrous hypergiant stars, which pump out millions of times more energy than the Sun. So just how big and powerful are hypergiant stars?

First, let’s take a look at a regular star like our Sun. Our Sun is the baseline, with 1 solar mass, and 1 solar diameter. It puts out 1 solar amount of luminosity. An example giant star would be Rigel, with 17 times the mass of the Sun. It’s putting out about 66,000 times as much energy as the Sun, and it’s estimated to have 62 times the radius of the Sun.

Next, let’s go bigger and look at a supergiant star: Betelgeuse. This familiar star is located in the constellation Orion, and has 20 times the mass of the Sun (1 solar mass = the mass of the Sun). Betelgeuse is estimated to be 1000 times the size of the Sun, and puts out 135,000 times as much energy.

Those stars are nothing compared to hypergiant stars. An example of a red hypergiant star is VY Canis Majoris, which measures 1,500 times the size of the Sun.

The true monsters of the Universe are the blue hypergiant stars, like Eta Carinae. It has 150 times the mass of the Sun, and measure up to 180 times the size of the Sun. Eta Carinae is putting out 4 million times as much energy as the Sun! Of course, Eta Carinae is a “live fast, die young” kind of star. It’s probably only been around for 3 million years or so, and astronomers think it’ll detonate as a supernova within 100,000 years.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about Eta Carinae, which is expected to blow up any time now.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://en.wikipedia.org/wiki/VY_Canis_Majoris
http://en.wikipedia.org/wiki/Rigel
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970616b.html
http://seds.org/messier/xtra/ngc/etacar.html

February 10, 2009December 24, 2015 by Nancy Atkinson

Cosmologists Look Back to Cosmic Dawn

The Universe 590 million years after the Big Bang. Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

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What did the Universe look like early in its history, only 500 million years after the Big Bang? Currently, we have no way of actually “looking” back that far with our telescopes, but cosmologists from Durham University in the UK have used a computer simulation to predict how the very early Universe would have appeared. The images portray the “Cosmic Dawn,” and calculate the formation of the first big galaxies. The simulation also attempts to discern the role that dark matter played in galaxy formation. “We are effectively looking back in time and by doing so we hope to learn how galaxies like our own were made and to understand more about dark matter,” said Alvaro Orsi, lead author of the study from Durham University’s Institute for Computational Cosmology (ICC). “The presence of dark matter is the key to building galaxies – without dark matter we wouldn’t be here today.”

In the images produced by the computer simulation, the green swirls represent dark matter, which the scientists say is an essential ingredient in galaxy formation, while the circles show the star formation rate in galaxies. The different color circles represent the varying luminosity of star formation with yellow being brightest. The top image portrays the Universe as it was 590 million years after the Big Bang, and the image below shows the Universe 1 billion years after the Big Bang, as star formation rates begin to ramp up.

The Universe 1 billion years after the Big Bang. Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

The very first galaxies were created from the debris of massive stars which died explosively shortly after the beginning of the Universe. The Durham calculation predicts where these galaxies appear and how they evolve to the present day, over 13 billion years later. Although the galaxies today are bigger, they are not forming stars as quickly now as they were in the past. “Our research predicts which galaxies are growing through the formation of stars at different times in the history of the Universe and how these relate to the dark matter,” said co-author Dr. Carlton Baugh. “We give the computer what we think is the recipe for galaxy formation and we see what is produced which is then tested against observations of real galaxies.”

The massive simulation shows how structures grow in dark matter with a model showing how normal matter, such as gas, behaves to predict how galaxies grow. Gas feels the pull of gravity from dark matter and is heated up before cooling by releasing radiation and turning into stars. The simulation images show which galaxies are forming stars most vigorously at a given time. The image below shows the Universe 1.9 billion years after the Big Bang, a very active time of star formations in galaxies.

The Universe 1.9 billion years after the Big Bang. Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

The calculations of the Durham team, supported by scientists at the Universidad Catolica in Santiago, Chile, can be tested against new observations reaching back to early stages in the history of the Universe almost one billion years after the Big Bang. Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, said: “Computational cosmology plays an important part in our understanding of the Universe. Not only do these simulations allow us to look back in time to the early Universe but they complement the work and observations of our astronomers.”

This image shows the Universe today, 13.6 billion years after the Big Bang. Galaxies are not forming stars as quickly now as they were in the past.

The Universe today. Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

The team hopes that further study and simulations of effects of dark matter on galaxies will help astronomers learn more about what this ubiquitous substance is.

Source: Science and Technology Facilities Council

Institute for Computational Cosmology, Durham University
Department of Physics, Durham University

February 10, 2009May 4, 2017 by Fraser Cain

How Long Do Stars Last?

Artist's impression of a red giant star. Image credit: ESO

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The mass of a star defines its lifespan. The least massive stars will live the longest, while the most massive stars in the Universe will use their fuel up in a few million years and end in a spectacular supernova explosion. So, how long do stars last?

There are factors that will define how long a star will survive; how quickly they burn through the hydrogen fuel in their cores, and whether they have any way to keep the fuel in their core mixed up. Our own Sun has three distinct layers, the core, where nuclear fusion takes place, the radiative zone, where photons are emitted and then absorbed by atoms in the star. The final zone is the convective zone. In this region, hot gas from the edge of the radiative zone is carried upwards to the surface of the star in columns of hot plasma.

Let’s star with the largest stars. The largest possible stars probably have 150 times the mass of the Sun; for example, the monster Eta Carinae located about 8,000 light years from here. Eta Carinae was probably formed less than 3 million years ago. It consumes fuel so fast in its core that it gives off 4 million times the energy of the Sun. Astronomers think that Eta Carinae has less than 100,000 years to live. In fact, it could detonate as a supernova any day now…

As stars get smaller, they live longer. Our own Sun has been around for 4.5 billion years, slowly turning hydrogen into helium at its core. The Sun will run out of this hydrogen fuel in another 5 billion year or so, and it will turn into a red giant. It will expand to many times its original size and then eject its outer layers and shrink down to a tiny white dwarf star, a dense object the size of the Earth. So the total lifespan of a star with the mass of the Sun is about 10 billion years.

The smallest stars are the red dwarfs, these start at 50% the mass of the Sun, and can be as small as 7.5% the mass of the Sun. A red dwarf with only 10% the mass of the Sun will emit 1/10,000th the amount of energy given off by the Sun. Furthermore, red dwarfs lack radiative zones around their cores. Instead, the convective zone of the star comes right down to the cure. This means that the core of the star is continuously mixed up, and the helium ash is carried away to prevent it from building up. Red dwarf stars use up all their hydrogen, not just the stuff in the core. It’s believed that the smaller red dwarf stars will live for 10 trillion years or more.

How long do stars last? The biggest stars last only millions, the medium-sized stars last billions, and the smallest stars can last trillions of years.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe. And here’s an article about how the Earth won’t survive when the Sun becomes a red giant.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
SEDS.org
University of California – Berkeley
NASA

February 10, 2009December 24, 2015 by Fraser Cain

History of Stars

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Ancient peoples first looked up thousands of years ago, and the stars were there; pinpoints of light that seemed to slowly rotate around the Earth. The first astronomers also noticed the planets, the Moon and the Sun, and their motions across the night sky. Let’s learn about the history of stars.

We now know that stars are hot balls of hydrogen and helium, with nuclear fusion at their core. They can live billions and even trillions of years, consuming their hydrogen fuel. But ancient peoples had no idea what they were.

But they’ve always been important. The stars played a part in religious ceremonies, and navigators used them to travel at night, both over land and at sea. Early astronomers grouped the stars into constellations, and then used these to track the movement of the Sun and the planets. The motions of the stars over the course of a full year helped them build the first accurate calendars, to know when to plant fields and when to harvest.

In 1584, Giordana Bruno proposed that stars were other objects like our Sun, just much further away. Astronomers then started measuring changes in the luminosity of stars, and even the proper motion of nearby stars; they had changed their position since they were first measured by the ancient Greek astronomers Ptolemy and Hipparchus. The first measurement of distance to star was made by Friedrich Bessell in 1838 using the parallax technique – 61 Cygnus was measured to be 11.4 light years away.

In the 20th century, astronomers finally started using photography to image stars, and techniques were developed to measure the spectra of light coming off them. Theoretical advances in physics helped explain the different colors of stars and how this matched their luminosity and temperature.

We now know that our Milky Way galaxy contains between 200 and 400 billion stars and that there could be as many as 500 billion galaxies out there with just as many stars. Individual stars are mostly seen in our galaxy, but they have been imaged as far away as 100 million light-years.

We have written many articles about stars here on Universe Today. Here’s an article about how many stars there are in the Milky Way.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://planetquest.jpl.nasa.gov/science/science_index.cfm
http://cosmology.carnegiescience.edu/timeline/1838
http://en.wikipedia.org/wiki/61_Cygni

February 10, 2009December 24, 2015 by Fraser Cain

Are there Green Stars?

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We know there are red stars and blue stars, and yellow/white stars like our own Sun, but are there green stars? What would it take to get a star be green?

As you probably know, the color of a star depends on the temperature of its surface. The coolest stars are red, and have a surface temperature of less than 3,500 Kelvin. The hottest stars are blue, and have temperatures above 12,000 Kelvin. Our own Sun gives off an almost purely white light, and it measures 6,800 Kelvin.

Stars can be give off light from every point of the spectrum: infrared, red, orange, yellow, green, blue, indigo, violet, and ultraviolet. Astronomers measure the light curve of the photons coming off a star. In other words, that’s the ratio of photons streaming from the star in every part of the spectrum. The hottest stars have their peak in the blue part of the spectrum, and the coolest stars peak in the red. An average star like our Sun actually peaks in the green part of the spectrum. There are more photons coming from our Sun in the green part of the spectrum, and yet it looks white.

The problem is that stars like our Sun cast off photons in so many colors that it all looks white from our perspective. In order to get a green star, you would need to have a light curve that peaks right at green, but doesn’t give off light in many other colors. And there aren’t any stars that can do that. If you make the star hotter, it just gets bluer. And if you make a star cooler, it just becomes orange and then redder. There’s no way to have a light curve that makes a star look green.

So no, there are no green stars.

There are, however, other objects in space that do look green. These give off enough photons in the green spectrum to overwhelm the other colors. But there aren’t many objects out there.

We have written many articles about stars here on Universe Today. Here’s an article about red stars, and here’s an article about blue stars.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.astronomy.ohio-state.edu/~ryden/ast162_2/notes8.html
http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

February 10, 2009December 24, 2015 by Fraser Cain

Glow in the Dark Stars

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Sometimes you’ve just got to bring the Universe home and set it up inside your house. One of my favorite teaching tools for my children are glow in the dark stars. These are luminescent plastic stars and planets that have a sticky adhesive so you can stick them to your ceiling and see constellations when the lights are out. Whether you decide to actually recreate the night sky accurately, or just stick up stars randomly, glow in the dark stars are good fun.

Probably the best source for glow in the dark stars is Amazon.com. They sell a variety of packages to fit any sized ceiling.

Here’s a set of 50 glowing mini-stars with sticky backs. I like the mini-stars better than the bigger ones because they look more like real stars. I wish they didn’t have points, though.

On the other end of the spectrum is this Star Explosion Glow in the Dark. It’s a huge box with more than 500 glow in the dark stars, galaxies, and planets. Some of the pieces are pretty big.

But if you’re going to put glow in the dark stars on your ceiling, do it right. This set, called Nightscapes, gives you glow in the dark paint to make the stars yourself. They give you accurate star charts and show you how to make different sized dots to match stars of different brightness. This is a way to really make the constellations on your ceiling.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

February 10, 2009May 4, 2017 by Fraser Cain

Interesting Facts About Stars

Think you know everything there is to know about stars? Think again! Here’s a list of 10 interesting facts about stars; some you might already know, and few that are going to be new.

1. The Sun is the closest star

Okay, this one you should know, but it’s pretty amazing to think that our own Sun, located a mere 150 million km away is average example of all the stars in the Universe. Our own Sun is classified as a G2 yellow dwarf star in the main sequence phase of its life. The Sun has been happily converting hydrogen into helium at its core for 4.5 billion years, and will likely continue doing so for another 7+ billion years. When the Sun runs out of fuel, it will become a red giant, bloating up many times its current size. As it expands, the Sun will consume Mercury, Venus and probably even Earth. Here are 10 facts about the Sun.

2. Stars are made of the same stuff

All stars begin from clouds of cold molecular hydrogen that gravitationally collapse. As they cloud collapses, it fragments into many pieces that will go on to form individual stars. The material collects into a ball that continues to collapse under its own gravity until it can ignite nuclear fusion at its core. This initial gas was formed during the Big Bang, and is always about 74% hydrogen and 25% helium. Over time, stars convert some of their hydrogen into helium. That’s why our Sun’s ratio is more like 70% hydrogen and 29% helium. But all stars start out with 3/4 hydrogen and 1/4 helium, with other trace elements.

3. Stars are in perfect balance

You might not realize but stars are in constant conflict with themselves. The collective gravity of all the mass of a star is pulling it inward. If there was nothing to stop it, the star would just continue collapsing for millions of years until it became its smallest possible size; maybe as a neutron star. But there is a pressure pushing back against the gravitational collapse of the star: light. The nuclear fusion at the core of a star generates a tremendous amount of energy. The photons push outward as they make their journey from inside the star to reach the surface; a journey that can take 100,000 years. When stars become more luminous, they expand outward becoming red giants. And when they run out of light pressure, they collapse down into white dwarfs.

4. Most stars are red dwarfs

If you could collect all the stars together and put them in piles, the biggest pile, by far, would be the red dwarfs. These are stars with less than 50% the mass of the Sun. Red dwarfs can even be as small as 7.5% the mass of the Sun. Below that point, the star doesn’t have the gravitational pressure to raise the temperature inside its core to begin nuclear fusion. Those are called brown dwarfs, or failed stars. Red dwarfs burn with less than 1/10,000th the energy of the Sun, and can sip away at their fuel for 10 trillion years before running out of hydrogen.

5. Mass = temperature = color

The color of stars can range from red to white to blue. Red is the coolest color; that’s a star with less than 3,500 Kelvin. Stars like our Sun are yellowish white and average around 6,000 Kelvin. The hottest stars are blue, which corresponds to surface temperatures above 12,000 Kelvin. So the temperature and color of a star are connected. Mass defines the temperature of a star. The more mass you have, the larger the star’s core is going to be, and the more nuclear fusion can be done at its core. This means that more energy reaches the surface of the star and increases its temperature. There’s a tricky exception to this: red giants. A typical red giant star can have the mass of our Sun, and would have been a white star all of its life. But as it nears the end of its life it increases in luminosity by a factor of 1000, and so it seems abnormally bright. But a blue giant star is just big, massive and hot.

6. Most stars come in multiples

It might look like all the stars are out there, all by themselves, but many come in pairs. These are binary stars, where two stars orbit a common center of gravity. And there are other systems out there with 3, 4 and even more stars. Just think of the beautiful sunrises you’d experience waking up on a world with 4 stars around it.

7. The biggest stars would engulf Saturn

Speaking of red giants, or in this case, red supergiants, there are some monster stars out there that really make our Sun look small. A familiar red supergiant is the star Betelgeuse in the constellation Orion. It has about 20 times the mass of the Sun, but it’s 1,000 times larger. But that’s nothing. The largest known star is the monster VY Canis Majoris. This star is thought to be 1,800 times the size of the Sun; it would engulf the orbit of Saturn!

8. The most massive stars are the shortest lived

I mentioned above that the low mass red dwarf stars can sip away at their fuel for 10 trillion years before finally running out. Well, the opposite is true for the most massive stars that we know about. These giants can have as much as 150 times the mass of the Sun, and put out a ferocious amount of energy. For example, one of the most massive stars we know of is Eta Carinae, located about 8,000 light-years away. This star is thought to have 150 solar masses, and puts out 4 million times as much energy. While our own Sun has been quietly burning away for billions of years, and will keep going for billions more, Eta Carinae has probably only been around for a few million years. And astronomers are expecting Eta Carinae to detonate as a supernovae any time now. When it does go off, it would become the brightest object in the sky after the Sun the Moon. It would be so bright you could see it during the day, and read from it at night.

9. There are many, many stars

Quick, how many stars are there in the Milky Way. You might be surprised to know that there are 200-400 billion stars in our galaxy. Each one is a separate island in space, perhaps with planets, and some may even have life. But then, there could be as many as 500 billion galaxies in the Universe, and each of which could have as many or more stars as the Milky Way. Multiply those two numbers together and you’ll see that there could be as many as 2 x 10²³ stars in the Universe. That’s 200,000,000,000,000,000,000,000.

10. And they’re very far

With so many stars out there, it’s amazing to consider the vast distances involved. The closest star to Earth is Proxima Centauri, located 4.2 light-years away. In other words, it takes light itself more than 4 years to complete the journey from Earth. If you tried to hitch a ride on the fastest spacecraft ever launched from Earth, it would still take you more than 70,000 years to get there from here. Traveling between the stars just isn’t feasible right now.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
NASA: How Do Stars Form and Evolve?
NASA: Stars

February 10, 2009December 24, 2015 by Fraser Cain

Giant Stars

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

75% of all the stars in the Universe are smaller and less massive than the Sun. Most of the others are similar in size and mass to the Sun, or maybe a little larger. But there are some very rare stars out there that are much larger and more massive than our Sun; these are the giant stars.

Blue Giant Stars
The color of a star depends on its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star depends entirely on its mass. If a star has enough mass, it will have a surface temperature greater than about 10,000 Kelvin and shine with a blue color. The largest and hottest stars in the Universe are these blue giant stars.

A familiar example is the blue giant star Rigel, located in the constellation of Orion, located about 700 to 900 light years away. Rigel contains 17 times the mass of the Sun, and shines with 40,000 times the luminosity of the Sun. This is enough energy for Rigel to light up dust clouds in its vicinity.

An even more extreme example is the blue hypergiant Eta Carinae, located about 8,000 light years away. Eta Carinae is a monster, estimated to have more than 100 times the mass of the Sun. It’s burning fuel at such a tremendous rate that it puts out 4 million times as much energy as the Sun, with a surface temperature of 40,000 Kelvin. Astronomers expect Eta Carinae to detonate as a supernova in a few hundred thousand years.

Blue giant stars are giant because they have many times the mass of the Sun.

Red Giant Stars
On the other end of the spectrum are the red giant stars. While blue is the hottest color of stars, red is the coolest color they can have. A red giant is born when a star like our Sun reaches the end of its life and runs out of hydrogen fuel in its core. This forces the star to begin nuclear fusion with helium, increase in luminosity and bloat up many times its original size. When our Sun becomes a red giant, it will expand to consume the orbits of the inner planets, including Mercury, Venus and Earth.

So, regular stars become regular red giants. But there are even larger red giants out there; the red supergiants. These are massive stars with more than 20 times the mass of the Sun. They enter the red giant phase of stellar evolution, but instead of merely expanding to the orbit of the Earth, they can expand to more than 1,500 times the radius of the Sun. Imagine a star that extended out past the orbit of Saturn.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about a planet surviving when its star became a red giant.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://en.wikipedia.org/wiki/Blue_giant
http://earthsky.org/brightest-stars/blue-white-rigel-is-orions-brightest-star
http://www.telescope.org/pparc/res8.html