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
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!

Red giant. Credit:NASA/ Walt Feimer
Red giant. Credit:NASA/ Walt Feimer

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.

Found! Sun’s ‘Sibling’ Likely Formed From Same Gas Cloud, Astronomers Say

Peer about 110 light-years away from our solar system, and you might catch a glimpse of how our own neighborhood came together. The recent discovery that HD 162826 — a star bright enough to be seen in binoculars — could be a “sibling” of our sun could shed more light on the solar system’s formation, astronomers said.

“We want to know where we were born,” stated Ivan Ramirez, an astronomer at the University of Texas at Austin who led the research. “If we can figure out in what part of the galaxy the sun formed, we can constrain conditions on the early solar system. That could help us understand why we are here.”

The star is called a “sibling” because it could have formed from the same gas and dust cloud in which our own solar system was formed, some 4.5 billion years ago. Since life is in our own solar system, a natural next question is whether HD 162826 could also have life-bearing planets. There is a tiny reason for “yes”, the astronomers said.

Basically, the argument goes that when the stars were first born and close together, chunks of matter could have been knocked off protoplanets and travelled between the two solar systems. There’s a small chance that this could have brought primitive life to Earth, although of course there’s a long way to go before that could even be proved.

This artist's conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Credit: University of Copenhagen/Lars Buchhave
This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Credit: University of Copenhagen/Lars Buchhave

That said, no planets have yet been found around HD 162826. (The star was known before, but just recently identified as a “sibling.”) Separate studies by the University of Texas and University of South Wales said there are likely no “hot Jupiters” (Jupiter-sized planets close to the star) nor Jupiter-sized planet in the solar system even further away. Smaller terrestrial planets, however, would have escaped the notice of this particular study.

The star is about 15 percent more massive than our sun and was selected from a list of 30 candidates based on its chemistry and orbit. There could also be more siblings out there to find, with one potential big help coming soon: the Gaia survey from the European Space Agency launched in December, which will chart the Milky Way in three dimensions.

Because Gaia will showcase the distance and motions of a billion stars, this will allow astronomers to look for these “solar siblings” as far in as the galaxy’s center, increasing the number of stars studied by a factor of 10,000. The exciting thing, the astronomers add, is with enough stars pinpointed as siblings to our sun, their orbits can then be traced back to the origin point — showing the location in the cosmos where the sun first came to be.

More information will be available in the June 1 issue of the Astrophysical Journal. A preprint version is available on Arxiv.

Source: University of Texas at Austin

Isotopes May One Day Aid In Planet Search

When we consider samples from the solar nebula, we think about comets and meteorites. These materials come from our solar system’s beginning, but the clues they give to formation don’t always mesh neatly. Thanks to a new study done by Carnegie’s Alan Boss, we’re now able to take a look at the Sun’s formation through a set of theoretical models. This work could not only help explain some of the differences we’ve discovered, but could also point to habitable exoplanets.

At the present time, a way to look back at the solar system’s early period is to theorize about tiny pockets of crystalline particles found in comets. These particles were forged at high temperatures. An alternate method of studying solar system formation is to analyze isotopes. These variants of elements carry the exact same number of protons, but contain a different number of neutrons. Unlike the crystalline particles, we can get our hands on samples of isotopes, because they are found in meteorites. As they decay, they turn into different elements. However, the initial number of isotopes can clue researchers as to their origin and how they might have journeyed across the neophyte solar system.

“Stars are surrounded by disks of rotating gas during the early stages of their lives.” says the Carnegie team. “Observations of young stars that still have these gas disks demonstrate that Sun-like stars undergo periodic bursts, lasting about 100 years each, during which mass is transferred from the disk to the young star.”

However, the study isn’t cut and dried just yet. The study of both particles and isotopes from comets and meteorites still present a somewhat confused look at early solar system formation. It would appear there’s more to the picture than just a single path of matter from the protoplanetary disk to the parent star. The crystalline grains found in comets are heat-formed and they signal that considerable mixing and outward flow occurred from materials close to the parent star and out to the perimeter of the system itself. Certain isotopes, such as aluminum, support this theory, but others, like oxygen, defy such a neat explanation.

According to the news release, Boss’ new model shows how a period of slight gravitational instability in the gas disk surrounding a proto-Sun about to go into an outburst phase, could account for these findings. What’s more, the models also predict this could happen with a wide variety of both mass and disk sizes. It shows that instability can “cause a relatively rapid transportation of matter between the star and the gas disk, where matter is moved both inward and outward. This accounts for the presence of heat-formed crystalline particles in comets from the solar system’s outer reaches.”

So what of aluminum? According to Boss’ model, the ratios of aluminum isotopes can be explained. It would appear the original isotope was imparted during a singular event – such as an exploding star sending a shock wave both inward and outward in the protoplanetary disk. As far as oxygen goes, it can be present in different pattern because it originated from sustained chemical reactions natural to the outer solar nebula and did not just happen as a singular event.

“These results not only teach us about the formation of our own solar system, but also could aid us in the search for other stars orbited by habitable planets,” Boss said. “Understanding the mixing and transport processes that occur around Sun-like stars could give us clues about which of their surrounding planets might have conditions similar to our own.”

Original Story Source: Carnegie Institution for Science Press Release