Friendly Giants Have Cozy Habitable Zones Too

It is an well-known fact that all stars have a lifespan. This begins with their formation, then continues through their Main Sequence phase (which constitutes the majority of their life) before ending in death. In most cases, stars will swell up to several hundred times their normal size as they exit the Main Sequence phase of their life, during which time they will likely consume any planets that orbit closely to them.

However, for planets that orbit the star at greater distances (beyond the system’s “Frost Line“, essentially), conditions might actually become warm enough for them to support life. And according to new research which comes from the Carl Sagan Institute at Cornell University, this situation could last for some star systems into the billions of years, giving rise to entirely new forms of extra-terrestrial life!

In approximately 5.4 billion years from now, our Sun will exit its Main Sequence phase. Having exhausted the hydrogen fuel in its 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, which in turn will cause the Sun to grow in size and enter what is known as the Red Giant-Branch (RGB) phase of its evolution.

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 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

This period will begin with our Sun becoming a subgiant, in which it will slowly double in size over the course of about half a billion years. It will then spend the next half a billion years expanding more rapidly, until it is 200 times its current size and several thousands times more luminous. It will then officially be a red giant star, eventually expanding to the point where it reaches beyond Mars’ orbit.

As we explored in a previous article, planet Earth will not survive our Sun becoming a Red Giant – nor will Mercury, Venus or Mars. But beyond the “Frost Line”, where it is cold enough that volatile compounds – such as water, ammonia, methane, carbon dioxide and carbon monoxide – remain in a frozen state, the remain gas giants, ice giants, and dwarf planets will survive. Not only that, but a massive thaw will set in.

In short, when the star expands, its “habitable zone” will likely do the same, encompassing the orbits of Jupiter and Saturn. When this happens, formerly uninhabitable places – like the Jovian and Cronian moons – could suddenly become inhabitable. The same holds true for many other stars in the Universe, all of which are fated to become Red Giants as they near the end of their lifespans.

However, when our Sun reaches its Red Giant Branch phase, it is only expected to have 120 million years of active life left. This is not quite enough time for new lifeforms to emerge, evolve and become truly complex (i.e. like humans and other species of mammals). But according to a recent research study that appeared in The Astrophysical Journal – titled “Habitable Zone of Post-Main Sequence Stars” – some planets may be able to remain habitable around other red giant stars in our Universe for much longer – up to 9 billion years or more in some cases!

Ramses Ramirez, left, and Lisa Kaltenegger hold a replica of our own habitable world, as they hunt for other places in the universe where life can thrive. Credit: Chris Kitchen/University Photo
Ramses Ramirez (left) and Lisa Kaltenegger are on the hunt for other places in the universe where life can thrive. Credit: Chris Kitchen/University Photo

To put that in perspective, nine billion years is close to twice the current age of Earth. So assuming that the worlds in question also have the right mix of elements, they will have ample time to give rise to new and complex forms of life. The study’s co-author, Professor Lisa Kaltennegeris, is also the director of the Carl Sagan Institute. As such, she is no stranger to searching for life in other parts of the Universe. As she explained to Universe Today via email:

“We found that planets – depending on how big their Sun is (the smaller the star, the longer the planet can stay habitable) – can stay nice and warm for up to 9 Billion years. That makes an old star an interesting place to look for life. It could have started sub-surface (e.g. in a frozen ocean) and then when the ice melts, the gases that life breaths in and out can escape into the atmosphere – what allows astronomers to pick them up as signatures of life. Or for the smallest stars, the time a formerly frozen planet can be nice and warm is up to 9 billion years. Thus life could potentially even get started in that time.”

Using existing models of stars and their evolution – i.e. one-dimensional radiative-convective climate and stellar evolutionary models – for their study, Kaltenegger and Ramirez were able to calculate the distances of the habitable zones (HZ) around a series of post-Main Sequence (post-MS) stars. Ramses M. Ramirez – a research associate at the Carl Sagan Institute and the lead author of the paper – explained the research process to Universe Today via email:

“We used stellar evolutionary models that tell us how stellar quantities, mainly the brightness, radius, and temperature all change with time as the star ages through the red giant phase. We also used a  climate model to then compute how much energy each star is outputting at the boundaries of the habitable zone. Knowing this and the stellar brightness mentioned above, we can compute the distances to these habitable zone boundaries.”

After several billions years, yellow suns (like ours) become Red Giants, expanding to several hundred times their normal size. Credit: Wendy Kenigsburg
After several billions years, yellow suns (like ours) become Red Giants, expanding to several hundred times their normal size. Credit: Wendy Kenigsburg

At the same time, they considered how this kind of stellar evolution could effect the atmosphere of the star’s planets. As a star expands, it loses mass and ejects it outward in the form of solar wind. For planets that orbit close to a star, or those that have low surface gravity, they may find some or all of their atmospheres blasted away. On the other hand, planets with sufficient mass (or positioned at a safe distance) could maintain most of their atmospheres.

“The stellar winds from this mass loss erodes planetary atmospheres, which we also compute as a function of time,” said Ramirez. “As the star loses mass, the solar system conserves angular momentum by moving outwards. So, we also take into account how the orbits move out with time.” By using models that incorporated the rate of stellar and atmospheric loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phases of star, they were able to determine how this would play out for planets that ranged in size from super-Moons to super-Earths.

What they found was that a planet can stay in a post-HS HZ for eons or more, depending on how hot the star is, and figuring for metallicities that are similar to our Sun’s. As Ramirez explained:

“The main result is that the maximum time that a planet can remain in this red giant habitable zone of hot stars is 200 million years. For our coolest star (M1), the maximum time a planet can stay within this red giant habitable zone is 9 billion years. Those results assume metallicity levels similar to those of our Sun. A star with a higher percentage of metals takes longer to fuse the non-metals (H, He..etc) and so these maximum times can increase some more, up to about a factor of two.”

Europa's cracked, icy surface imaged by NASA's Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute.
Could Europa’s cracked, icy surface thaw and give rise to a new habitable world when our Sun becomes a Red Giant in a few billion years? Credit: NASA/JPL-Caltech/SETI Institute

Within the context of our Solar System, this could mean that in a few billion years, worlds like Europa and Enceladus (which are already suspected of having life beneath their icy surfaces) might get a shot at becoming full-fledged habitable worlds. As Ramirez summarized beautifully:

“This means that the post-main-sequence is another potentially interesting phase of stellar evolution from a habitability standpoint. Long after the inner system of planets have been turned into sizzling wastelands by the expanding, growing red giant star, there could be potentially habitable abodes farther away from the chaos. If they are frozen worlds, like Europa, the ice would melt, potentially unveiling any preexisting life. Such pre-existing life may be detectable by future missions/telescopes looking for atmospheric biosignatures.”

But perhaps the most exciting take-away from their research study was their conclusion that planets orbiting within their star’s post-MS habitable zones would be doing so at distances that would make them detectable using direct imaging techniques. So not only are the odds of finding life around older stars better than previously thought, we should have no trouble in spotting them using current exoplanet-hunting techniques!

It is also worth noting that Kaltenegger and Dr. Ramirez have submitted a second paper for publication, in which they provide a list of 23 red giant stars within 100 light-years of Earth. Knowing that these stars, all of which are in our stellar neighborhood, could have life-sustaining worlds within their habitable zones should provide additional opportunities for planet hunters in the coming years.

And be sure to check out this video from Cornellcast, where Prof. Kaltenegger shares what inspires her scientific curiosity and how Cornell’s scientists are working to find proof of extra-terrestrial life.

Further Reading: The Astrophysical Journal

Living with a Capricious Star: What Drives the Solar Cycle?

You can be thankful that we bask in the glow of a relatively placid star. Currently about halfway along its 10 billion year career on the Main Sequence, our Sun fuses hydrogen into helium in a battle against gravitational collapse. This balancing act produces energy via the proton-proton chain process, which in turn, fuels the drama of life on Earth.

Looking out into the universe, we see stars that are much more brash and impulsive, such as red dwarf upstarts unleashing huge planet-sterilizing flares, and massive stars destined to live fast and die young.

Our Sun gives us the unprecedented chance to study a star up close, and our modern day technological society depends on keeping a close watch on what the Sun might do next. But did you know that some of the key mechanisms powering the solar cycle are still not completely understood?

Image credit: David Dickinson
One of the exceptionally active sunspot groups seen for Cycle #24 in early 2014. Image credit: David Dickinson

One such mystery confronting solar dynamics is exactly what drives the periodicity related to the solar cycle. Follow our star with a backyard telescope over a period of years, and you’ll see sunspots ebb and flow in an 11 year period of activity. The dazzling ‘surface’ of the Sun where these spots are embedded is actually the photosphere, and using a small telescope tuned to hydrogen-alpha wavelengths you can pick up prominences in the warmer chromosphere above.

This cycle is actually is 22 years in length (that’s 11 years times two), as the Sun flips polarity each time. A hallmark of the start of each solar cycle is the appearance of sunspots at high solar latitudes, which then move closer to the solar equator as the cycle progresses. You can actually chart this distribution in a butterfly diagram known as a Spörer chart, and this pattern was first recognized by Gustav Spörer in the late 19th century and is known as Spörer’s Law.

Sunspot_butterfly_graph
The ‘Butterfly diagram’ of sunspot distribution by latitude over previous solar cycles. Image credit: NASA/Marshall Spaceflight Center

We’re currently in the midst of solar cycle #24, and the measurement of solar cycles dates all the way back to 1755. Galileo observed sunspots via projection (the tale that he went blind observing the Sun in apocryphal). We also have Chinese records going back to 364 BC, though historical records of sunspot activity are, well, spotty at best. The infamous Maunder Minimum occurred from 1645 to 1717 just as the age of telescopic astronomy was gaining steam. This dearth of sunspot activity actually led to the idea that sunspots were a mythical creation by astronomers of the time.

But sunspots are a true reality. Spots can grow larger than the Earth, such as sunspot active region 2192, which appeared just before a partial solar eclipse in 2014 and could be seen with the unaided (protected) eye. The Sun is actually a big ball of gas, and the equatorial regions rotate once every 25 days, 9 days faster than the rotational period near the poles. And speaking of which, it is not fully understood why we never see sunspots at the solar poles, which are tipped 7.25 degrees relative to the ecliptic.

Other solar mysteries persist. One amazing fact about our Sun is the true age of the sunlight shining in our living room window. Though it raced from the convective zone and through the photosphere of the Sun at 300,000 km per second and only took 8 minutes to get to your sunbeam-loving cat here on Earth, it took an estimated 10,000 to 170,000 years to escape the solar core where fusion is taking place. This is due to the terrific density at the Sun’s center, over seven times that of gold.

Another amazing fact is that we can actually model the happenings on the farside of the Sun utilizing a new fangled method known as helioseismology.

Another key mystery is why the current solar cycle is so weak… it has even been proposed that solar cycle 25 and 26 might be absent all together. Are there larger solar cycles waiting discovery? Again, we haven’t been watching the Sun close enough for long enough to truly ferret these ‘Grand Cycles’ out.

Solar cycle
The sunspot number predicted for the current Cycle #24 versus reality. Image credit: NASA

Are sunspot numbers telling us the whole picture? Sunspot numbers are calculated using formula that includes a visual count of sunspot groups and the individual sunspots in them that are currently facing Earthward, and has long served as the gold standard to gauge solar activity. Research conducted by the University of Michigan in Ann Arbor in 2013 has suggested that the orientation of the heliospheric current sheet might actually provide a better picture as to the goings on of the Sun.

Another major mystery is why the Sun has this 22/11 year cycle of activity in the first place. The differential rotation of the solar interior and convective zone known as the solar tachocline drives the powerful solar dynamo.  But why the activity cycle is the exact length that it is is still anyone’s guess. Perhaps the fossil field of the Sun was simply ‘frozen’ in the current cycle as we see it today.

There are ideas out there that Jupiter drives the solar cycle. A 2012 paper suggested just that. It’s an enticing theory for sure, as Jupiter orbits the Sun once every 11.9 years.

The motion of the solar barycenter through the last half of the 20th century. Image credit: Carl Smith/Wikimedia Commons
The motion of the solar barycenter through the last half of the 20th century. Image credit: Carl Smith/Wikimedia Commons

And a recent paper has even proposed that Uranus and Neptune might drive much longer cycles…

Color us skeptical on these ideas. Although Jupiter accounts for over 70% of the planetary mass in the solar system, it’s 1/1000th as massive as the Sun. The barycenter of Jupiter versus the Sun sits 36,000 kilometres above the solar surface, tugging the Sun at a rate of 12.4 metres per second.

Rigs to view the Sun in both hydrogen-alpha and visible light. Credit: David Dickinson
Rigs to view the Sun in both hydrogen-alpha and visible light. Credit: David Dickinson

I suspect this is a case of coincidence: the solar system provides lots of orbital periods of varying lengths, offering up lots of chances for possible mutual occurrences. A similar mathematical curiosity can be seen in Bode’s Law describing the mathematical spacing of the planets, which to date, has no known basis in reality. It appears to be just a neat play on numbers. Roll the cosmic dice long enough, and coincidences will occur. A good test for both ideas would be the discovery of similar relationships in other planetary systems. We can currently detect both starspots and large exoplanets: is there a similar link between stellar activity and exoplanet orbits? Demonstrate it dozens of times over, and a theory could become law.

That’s science, baby.

What is the Life Cycle of Stars?

Much like any living being, stars go through a natural cycle. This begins with birth, extends through a lifespan characterized by change and growth, and ends in death. Of course, we’re talking about stars here, and the way they’re born, live and die is completely different from any life form we are familiar with.

For one, the timescales are entirely different, lasting on the order of billions of years. Also, the changes they go through during their lifespan are entirely different too. And when they die, the consequences are, shall we say, much more visible? Let’s take a look at the life cycle of stars.

Molecular Clouds:

Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating in a galaxy for millions of years, but then some event causes it to begin collapsing down under its own gravity. For example when galaxies collide, regions of cold gas are given the kick they need to start collapsing. It can also happen when the shockwave of a nearby supernova passes through a region.

As it collapses, the interstellar cloud breaks up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star. As the cloud collapses, the gravitational energy causes it to heat up, and the conservation of momentum from all the individual particles causes it to spin.

Protostar:

As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a protostar. Surrounding the protostar is a circumstellar disk of additional material. Some of this continues to spiral inward, layering additional mass onto the star. The rest will remain in place and eventually form a planetary system.

Depending on the stars mass, the protostar phase of stellar evolution will be short compared to its overall life span. For those that have one Solar Mass (i.e the same mass as our Sun), it lasts about 1000,000 years.

T Tauri Star:

A T Tauri star begins when material stops falling onto the protostar, and it’s releasing a tremendous amount of energy. They are so-named because of the prototype star used to research this phase of solar evolution – T Tauri, a variable star located in the direction of the Hyades cluster, about 600 light years from Earth.

A T Tauri star may be bright, but this all comes its gravitational energy from the collapsing material. The central temperature of a T Tauri star isn’t enough to support fusion at its core. Even so, T Tauri stars can appear as bright as main sequence stars. The T Tauri phase lasts for about 100 million years, after which the star will enter the longest phase of its development – the Main Sequence phase.

Main Sequence:

Eventually, the core temperature of a star will reach the point that fusion its core can begin. This is the process that all stars go through as they convert protons of hydrogen, through several stages, into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main sequence star releases a tremendous amount of energy.

This energy starts out as gamma rays in the core of the star, but as it takes a long slow journey out of the star, it drops down in wavelength. All of this light pushes outward on the star, and counteracts the gravitational force pulling it inward. A star at this stage of life is held in balance – as long as its supplies of hydrogen fuel lasts.

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 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

And how long does it last? It depends on the mass of the star. The least massive stars, like red dwarfs with half the mass of the Sun, can sip away at their fuel for hundreds of billions and even trillions of years. Larger stars, like our Sun will typically sit in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years.

Red Giant:

Over the course of its life, a star is converting hydrogen into helium at its core. This helium builds up and the hydrogen fuel runs out. When a star exhausts its fuel of hydrogen at its core, its internal nuclear reactions stop. Without this light pressure, the star begins to contract inward through gravity.

This process heats up a shell of hydrogen around the core which then ignites in fusion and causes the star to brighten up again, by a factor of 1,000-10,000. This causes the outer layers of the star to expand outward, increasing the size of the star many times. Our own Sun is expected to bloat out to a sphere that reaches all the way out to the orbit of the Earth.

The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Once a star reaches this point, it contracts down and is no longer a red giant. Stars much more massive than our Sun can continue on in this process, moving up the table of elements creating heavier and heavier atoms.

White Dwarf:

A star with the mass of our Sun doesn’t have the gravitational pressure to fuse carbon, so once it runs out of helium at its core, it’s effectively dead. The star will eject its outer layers into space, and then contract down, eventually becoming a white dwarf. This stellar remnant might start out hot, but it has no fusion reactions taking place inside it any more. It will cool down over hundreds of billions of years, eventually becoming the background temperature of the Universe.

We have written many articles about the live cycle of stars on Universe Today. Here’s What is the Life Cycle Of The Sun?, What is a Red Giant?, Will Earth Survive When the Sun Becomes a Red Giant?, What Is The Future Of Our Sun?

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?, Episode 13: Where Do Stars Go When they Die?, and Episode 108: The Life of the Sun.

Sources:

What Are The Different Types of Stars?

A star is a star, right? Sure there are some difference in terms of color when you look up at the night sky. But they are all basically the same, big balls of gas burning up to billions of light years away, right?  Well, not exactly. In truth, stars are about as diverse as anything else in our Universe, falling into one of many different classifications based on its defining characteristics.

All in all, there are many different types of stars, ranging from tiny brown dwarfs to red and blue supergiants. There are even more bizarre kinds of stars, like neutron stars and Wolf-Rayet stars. And as our exploration of the Universe continues, we continue to learn things about stars that force us to expand on the way we think of them. Let’s take a look at all the different types of stars there are.

Protostar:

A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

Size chart showing our Sun (far left) compared to larger stars. Credit: earthspacecircle.blogspot.ca
Size chart showing our Sun (far left) compared to larger stars. Credit: earthspacecircle.blogspot.ca

T Tauri Star:

A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Star:

The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.

A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.

The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.

Red Giant Star:

When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.

White Dwarf Star:

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more. A white dwarf will just cool down until it because the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

Red Dwarf Star:

Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.

Neutron Stars:

If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

Supergiant Stars:

The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.

As you can see, stars come in many sizes, colors and varieties. Knowing what accounts for this, and what their various life stages look like, are all important when it comes to understanding our Universe. It also helps when it comes to our ongoing efforts to explore our local stellar neighborhood, not to mention in the hunt for extra-terrestrial life!

We have written many articles about stars on Universe Today. Here’s What is the Biggest Star in the Universe?, What is a Binary Star?, Do Stars Move?, What are the Most Famous Stars?, What is the Brightest Star in the Sky, Past and Future?

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?