Author: Fraser Cain

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Small stars release small amounts of energy, and huge stars release tremendous amounts of energy. Astronomers refer to the amount energy coming off the surface of a star as “luminosity”.

As a baseline, astronomers measure the luminosity of other stars against the power of the Sun. So here’s the luminosity of the Sun: 3.839 x 10²⁶ watts, or 3.839 x 10³³erg/s. When you use this number, you can calculate how much of that energy hits the Earth, or would be visible from a specific distance.

To be able to calculate the luminosity of a star, there are three variables at play: distance, apparent magnitude, and visible luminosity. If you have two of those variables, you can always calculate the third.

Let’s take a look at the smallest, least luminous stars out there: red dwarfs. A red dwarf can be as small as 7.5% the mass of the Sun, and up to 50% of the Sun’s mass. An average red dwarf has 1/10,000th the luminosity of our Sun.

Red giants, on the other hand, look cool and red, but you’ve got to remember that they’re huge. A typical red giant in our Solar System would engulf the orbit of the Earth. A red giant may be releasing 1,000-10,000 times the luminosity of the Sun. The largest known red giant is about 1,800 times larger than the diameter of the Sun. It emits about 430,000 times as much energy as the Sun.

The most energetic stars out there are the blue stars. A fairly familiar blue star is Rigel, located in the constellation Orion. Rigel has 17 times the mass of the Sun, is located about 800 light years away. Its surface temperature is at least 11,000 Kelvin. And this blue giant star is putting out about 40,000 times as much energy as the Sun.

But perhaps the most energetic stars in the Universe are the blue hypergiants. The best example is Eta Carinae. This monster is thought to have 150 times the mass of the Sun. It’s surface temperature is about 40,000 Kelvin, and it’s thought to be blasting out more than a million times the energy of the Sun.

We have written many articles about stars here on Universe Today. Here’s an article about how Eta Carinae should detonate as a supernova soon, and here’s an article about VY Canis Majoris.

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?

References:
Wikipedia: Solar Luminosity
Wikipedia: Red Dwarf
Wikipedia: Rigel
Wikipedia: List of Most Massive Stars

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You might be surprised to know that the color of stars depends on their temperature. The coolest stars will look red, while the hottest stars will appear blue. And what defines the temperature of a star? It all comes down to mass.

The most common stars in the Universe are the relatively tiny red dwarf stars. These stars can have as little as 7.5% the mass of the Sun, and top out at about 50%. Red dwarfs use their stores of hydrogen fuel very slowly; it’s believed that a red dwarf star with about 10% the mass of the Sun may live for 10 trillion years or more. Our own Sun will only live for about 12 billion years. Red dwarf stars have a surface temperature of less than 3,500 Kelvin, and this is why they appear red to our eyes.

Our own Sun is classified as a yellow dwarf star. It has a surface temperature of about 5,800 Kelvin. Because of this temperature, the bulk of the light we see streaming from the Sun is yellow/white. Our Sun has been in the main sequence phase of its life for 4.5 billion years, and it’s expected to last another 7 billion years or so.

The hottest stars are the blue stars. These start at temperatures of about 10,000 Kelvin, and the biggest, hottest blue supergiants can be more than 40,000 Kelvin. In fact, there’s so much energy coming off the surface of a blue star that many could actually be classified as ultraviolet stars, it’s just that our eyes can’t see that high into the spectrum.

We have written many articles about stars here on Universe Today. Here’s an article about how red dwarf stars could have habitable zones, and here’s an article about how red dwarfs can clear out their dusty disks.

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?

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Astronomers now know that the Big Bang occurred 13.7 billion years ago. For the first few hundred million years, the entire Universe was too hot any stars to form. But then the Universe cooled down to the point that gravity could start pulling together the raw hydrogen and helium into the first ever stars.

The basic elements on the Universe, hydrogen and helium and a few trace elements, we formed during the Big Bang. For a brief moment, the entire Universe was at the temperature and pressure that hydrogen could fuse into helium. This is why we see roughly the same ratios of hydrogen to helium, everywhere we look in the Universe: 73% hydrogen, 25% helium, and the rest are trace elements.

Astronomers think that this pure hydrogen/helium mix allowed the first stars to grow much more massive than stars can get today. It’s believed that they could have gathered together several hundred solar masses. The most massive star that can form today is thought to only be about 150 solar masses. After that point, extreme winds coming from the star prevent any additional material from falling in.

This first generation of stars, which astronomers call Population III stars, would have lived short violent lives. They probably lasted just a million years or so, and then detonated as supernovae. But in their lives, these Population III stars would have created heavier and heavier elements at their cores, and in their violent deaths, they would have created the even more exotic heavier elements, like gold and uranium. It’s possible that the first stars went through a few quick cycles, pulling in material, detonating and seeing the region with heavier elements. Eventually the first long-term stars would have gotten going, stars with the amount of heavier elements we see today.

None of the first stars have ever been observed directly. There have been a few hints through gravitational lensing; using a nearby galaxy’s gravity to focus the light from a more distant quasar. The next generation of space telescopes, like the James Webb Space Telescope might be able to push the observable Universe back to these first stars.

We have written many articles about stars here on Universe Today. Here’s an article about astronomers simulating the formation of the first stars, and here’s an article about how the first stars could have been powered by dark matter.

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?

Source: Caltech IPAC

The biggest stars in the Universe are the red supergiant stars. And we’re talking really, really big. The largest known red supergiant is thought to be VY Canis Majoris, measuring about 1800 times the size of the Sun. Imagine if the Sun extended out to the orbit of Saturn. Let’s take a look at where red supergiant stars come from.

Red supergiants are similar to red giants. They form when a star runs out of hydrogen fuel in their core, begins collapsing, and then outer shells of hydrogen around the core get hot enough to begin fusion. While a red giant might form when a star with the mass of our Sun runs out of fuel, a red supergiant occurs when a star with more than 10 solar masses begins this phase.

The five largest known supergiants in the galaxy are red supergiants: VY Canis Majoris, Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni. Each of these stars has a radius larger than 1500 times the size of the Sun. In comparison, regular red giant is only 200 to 800 times the size of the Sun.

Red supergiant stars don’t last long; typically only a few hundred thousand years, maybe up to a million. Within this period, the core of the red supergiant continues to fuse heavier and heavier elements. This process stops when iron builds up in the core of the star. Iron is the equivalent of ash when it comes to nuclear fusion. The process of fusing iron actually requires more energy than it releases.

At this point, many red supergiants will detonate as Type II supernovae.

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 that talks about the three largest stars discovered.

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?

When star like our Sun reaches the end of its life, it enters one last phase, ballooning up to many times its original size. Astronomers call these objects red giant star, and you’ll want to learn more about them, since this is the future fate for the Sun. Don’t panic, we’ve got another 7 billion years or so before the Sun becomes a red giant star.

As you probably know, stars shine because they’re converting hydrogen into helium in their cores through a process called nuclear fusion. Our own Sun has been performing fusion at its core for 4.5 billion years, and will continue to do so for another 7 billions years, at least. The helium byproduct from this fusion reaction slowly builds up in the core of a star, and they have no way to get rid of it. Eventually, billions of year down the road, a star uses up the last of its hydrogen fuel.

Once a star exhausts this fuel source, it no longer has the outward light pressure to counteract the gravity pulling in on itself. And so, the star begins to collapse. Before the star can collapse too far, though, this contraction heats up a shell of hydrogen around the core of the star to the point that it can support nuclear fusion. The higher temperatures lead to increasing reaction rates, and the star’s energy output increases by a factor of 1000 to 1000x. This new extreme light pressure pushes out the star’s outer layers beginning its life as a red giant star.

A red giant will expand outward many times its original size. Our own Sun, for example will grow so large that it engulfs the orbits of Mercury, Venus and even Earth; although, it’s not certain if Earth will actually be destroyed when this happens.

The core of the star will become so hot and dense that the leftover helium fuel will no able to star fusing into heavier elements. Stars with the mass of our Sun will stop with helium, but more massive stars will keep going, fusing carbon and even heavier elements together.

Without any more fuel to burn, these stars will expel their outer layers and then contract down to become white dwarfs.

We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant. And were you wondering if the Earth will survive when the Sun becomes a red giant?

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?

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A star will live the majority of its live in the main sequence phase. This is where nuclear fusion of hydrogen into helium is happening in its core, and the light pressure of this energy balances out the gravitational collapse of the star. Before a star gets into the main sequence phase, though, it spends some time as a protostar – a baby star.

Stars form when vast clouds of cold molecular hydrogen and helium collapse under mutual gravity. This collapse could have been triggered by a galaxy collision, or the shockwave of a nearby supernova. As the cloud collapses, it breaks into fragments, each of which will eventually become a star of some size.

As the cloud contracts, it begins to increase in temperature. This comes from the conversion of gravitational energy into kinetic energy. The cloud continues to heat up, and the conservation of momentum of all the different particles causes the protostar to spin.

The collapse of the cloud happens fastest at its center, where the material is at the highest density and hottest temperature. Unfortunately these objects are shrouded in dust, and impossible to see with Earth-based observatories. They can be seen in infrared telescopes though, which can pierce through the veil of dust that shrouds them.

As the collapse continues, a disk of gas forms around the protostar, and bi-polar jets blast out from the top and bottom of the star. These produce spectacular shock waves in the clouds.

An object can be considered a protostar as long as material is still falling inward. After about 100,000 years or so, the protostar stops growing and the disk of material surrounding it is destroyed by radiation. It then becomes a T Tauri star, and is visible to Earth-based telescopes.

We have written many articles about stars on Universe Today. Here’s one article about protostars, and here’s another.

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?

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Most of the stars in the Universe are in the main sequence stage of their lives, a point in their stellar evolution where they’re converting hydrogen into helium in their cores and releasing a tremendous amount of energy. Let’s example the main sequence phase of a star’s life and see what role it plays in a star’s evolution.

A star first forms out of a cold cloud of molecular hydrogen and helium. Mutual gravity pulls the stellar material together, and this gravitational energy heats it up. The star first goes through a protostar phase for about 100,000 years, and then a T Tauri phase, where it shines only with the energy released from its ongoing gravitational collapse. This second T Tauri phase lasts a further 100 million years or so.

Eventually temperatures and pressures in the core of the star are sufficient that it can ignite nuclear fusion, converting hydrogen atoms into helium. When this process gets going, a star is said to be in the main sequence phase of its life.

In a star like our Sun, the core accounts for about 20% of its radius. It’s inside this region where all the energy of the Sun is released. The energy released in the core must then travel slowly through a radiative zone, where photons of energy are absorbed and then re-emitted. Energy is then carried through a convective zone, where columns of hot plasma carry bubbles of heated gas to the surface of the Sun where it’s released. The material cools down and falls back down inside the Sun where it’s heated up again. This journey can take more than 100,000 years for a single photon to get from the core of a star out to its surface.

Over time, a star slowly uses up the supply of hydrogen in its core, and leftover helium builds up. But the main sequence phase can last a long time. Our Sun has already been in its main sequence for 4.5 billion years, and will probably last another 7.5 billion years before it runs out of fuel.

The smallest red dwarf stars can smolder in the main sequence phase for an estimated 10 trillion years! The largest supergiant stars might only last a few million. It all comes down to mass.

And mass defines how a star comes out of the main sequence phase of its life. For the smallest red dwarf stars, astronomers think they’ll just shut off once they’ve used up all their hydrogen, becoming white dwarfs. More massive stars, with up to 10 solar masses, will go through a red giant phase where they expand many times their original size before collapsing down to the white dwarf. And the most massive stars will just explode as supernovae.

We have written many articles about stars on Universe Today. Here’s an article about the entire life cycle of stars, and different types of stars.

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?

References:
NASA
http://burro.astr.cwru.edu/stu/stars_lifedeath.html