Author: Fraser Cain

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Astronomers classify stars into groups according to their color and the presence of elements in the stars’ spectral signatures. This star classification system goes like this: O, B, A, F, G, K, M (here’s a way to remember them: “Oh be a fine girl, kiss me”.) M stars are coolest and most common stars in the Universe.

M stars range in temperature from 2,500 Kelvin and go all the way up to 3,500 Kelvin. They look red to our eyes. M stars account for 75% of the stars in our stellar neighborhood, so they’re the most common by far! Most M stars are tiny red dwarfs, with less than 50% of the mass of the Sun, but some are actually giants and supergiants, like the red giant Betelgeuse.

Some familiar M stars include Betalgeuse (red giant), and the red dwarfs Proxima Centauri, Barnard’s star, and Gliese 581

We have written many articles about stars here on Universe Today. Here’s an article about how red dwarf stars have small habitable zones.

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?

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To organize all the stars in the Universe, Astronomers use a classification system that collects the stars into groups based on their color and the presence of various elements in the star’s outer atmosphere. So, here are the classifications: O, B, A, F, G, K, M (if you need to remember then, just keep this in mind: “Oh be a fine girl, kiss me”.) K stars are cooler than the Sun.

K stars start at about 3,500 Kelvin, and can get as hot as 5,000 Kelvin. This makes them look orange-red to our eyes. K stars can actually vary in size from main sequence stars with less mass than the Sun to red giants and supergiants with many times the mass of the Sun. It’s all because of the temperature. They have weak hydrogen lines and mostly neutral metals, like Manganese, Iron and Silicon. About 13% of stars in the stellar neighborhood are K stars.

Some familiar K stars include Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran

We have written many articles about stars here on Universe Today. Here’s an article about the closest known star with extrasolar planets, Epsilon Eridani.

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?

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Astronomers collect the stars in the Universe into a classification system that organized them by color and spectral signature (the presence of various metals in the star’s outer atmosphere). Here are the classifications: O, B, A, F, G, K, M (if you need to remember then, just keep this in mind: “Oh be a fine girl, kiss me”.) G stars are perhaps the best known stars out there. That’s because our own Sun is a G star.

G stars range in temperature from 5,000 Kelvin to 6,000 Kelvin, and they appear white or yellow-white to our eyes. You can also recognize a G star by the presence of Calcium in their spectral signature, but with weaker hydrogen lines than F type stars. G stars represent 7.7% of all the stars in our stellar neighbourhood.

Some familiar G stars include The Sun, Alpha Centauri A, Capella, Tau Ceti

We have written many articles about stars here on Universe Today. Here’s an article about the search for planets around Alpha Centauri.

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?

Astronomers classify the stars out there into groups based on the color of the star and the presence of certain elements in the star’s atmosphere. The classifications are: O, B, A, F, G, K, M (just remember this handy mnemonic , “Oh be a fine girl, kiss me”.) F stars are still hotter than the Sun, appearing white to our eyes.

F stars have a surface temperature of 6,000 Kelvin to 7,200 Kelvin. You can also recognize an F star by the presence of Calcium in their spectral signature, as well as neutral metals like Iron and Chromium. F stars represent 3.1% of all stars.

Some familiar F stars include Arrakis, Canopus, Procyon.

We have written many articles about stars here on Universe Today. Here’s an article about some strange observations of Procyon.

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?

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Astronomers have developed a star classification system to organize all the stars we can see in the Universe; it’s based on color and the spectral signature of certain elements in the star’s atmosphere. The classifications are: O, B, A, F, G, K, M (here’s a handy mnemonic , “Oh be a fine girl, kiss me”.) A stars are some of the more common stars seen with the unaided eye: they appear white or bluish-white.

The surface temperatures of A stars range from 7,400 Kelvin to 10,000 Kelvin; that’s about twice the temperature of the Sun, so these stars are really hot. Astronomers also recognize them by the strong hydrogen lines, as well as lines of ionized metals, like Iron, Magnesium and Silicon. A stars are more massive than the Sun, but don’t lead lives that are too much different than the life of our own Sun.

Some familiar A stars include Vega, Sirius, and Deneb.

We have written many articles about stars here on Universe Today. Here’s an article about how Vega has a cool, dark equator, and it might even have planets.

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?

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Astronomers have developed a method of classifying stars based on their color and some other characteristics. The star classifications are O, B, A, F, G, K, M (you can remember that with the handy mnemonic, “Oh be a fine girl, kiss me”.) O stars are the most extreme group of all. They have the highest temperatures, the most luminosity, and the most mass (oh, and the shortest lives).

An O star appears blue to the eye, and can have a surface temperature of more than 41,000 Kelvin; its color would be better described as ultraviolet, but we can’t see that color with our eyes. The surface temperature of an O star is so great that hydrogen on the surface of the star is completely ionized, but other elements are more visible, like Helium, Oxygen, Nitrogen, and Silicon.

O stars are very massive and evolve very rapidly. Shortly after they form as a protostar, they already have the pressure and temperatures in their cores to begin hydrogen burning. The O stars light up their stellar nurseries with ultraviolet light and cause the clouds of nebula to glow. You can thank O stars for illuminating the beautiful nebula photographs captured by Hubble. O stars burn through their fuel quickly, and can detonate as supernovae in just a few million years.

Some O stars include Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis.

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

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?

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Although they’re just hot balls of hydrogen and helium, stars are constantly changing over time. Studying star evolution is a whole branch of astronomy, and scientists are learning new things all the time.

To really understand star evolution, you’ve got to go right back to the beginning. All the stars we see today started out as large clouds of cold molecular hydrogen. Some event, like a nearby supernova, passed through the cloud of gas and gave it the kick it needed to begin collapsing. The gravity of the cloud pulled unevenly, and so it tore into smaller clouds, each of which would go on to form a new star.

In one cloud, the material streamed together to form a growing ball of hydrogen and helium. This protostar was enshrouded in gas and dust, and would actually be invisible from our Earth-based telescopes. As the ball grew, more and more material came in, causing the protostar to spin, and releasing jets of material from its poles. This accumulation of material takes about 100,000 years.

Once all the material was accumulated, the pre-star was hot and glowing; almost like a real star. But it wasn’t heated by fusion reactions in its core, but through the gravitational energy of the continuously collapsing material. This hot, young object is known as a T Tauri star, and remains in this state for about 100 million years.

Finally the temperature and pressure at the core of the star were sufficient to allow nuclear fusion to get going. Now the star would become a true main sequence star, converting hydrogen into helium at its core. A star with the mass of our Sun could stay in the main sequence stage for more than 12 billion years. More massive stars will last for shorter periods of time, while the tiny red dwarf stars can last for hundreds of billions and even trillions of years.

Eventually the star runs out of hydrogen fuel in its core. Without the outward light pressure from the fusion reactions, the star starts to contract, creating more temperature and pressure in the core. A shell of hydrogen around the core can now undergo nuclear fusion, and so it does, increasing the star’s brightness hundreds and even thousands of times. And in the core of the star, helium is fused into even heavier elements. This causes the star to bloat out to become a red giant. Regular stars like our Sun will expand to the point that they consume the interior planets: Mercury, Venus and even Earth. Stars with more than 20 times the mass of the Sun become red supergiants, expanding out more than 1500 times the radius of the Sun. Imagine a star so big it consumed the orbit of Saturn!

This extra fuel runs out and so the star collapses down on itself again. More massive stars will be able to do this trick multiple times, burning new shells and burning heavier and heavier elements. Eventually all stars reach their limit. The most massive stars, those with more than 20 times the mass of the Sun, will detonate as supernovae. Less massive stars will eject their outer layers and then collapse inward forming a white dwarf, neutron star or black hole. Our Sun will form a white dwarf; a remnant the size of the Earth with 60% of its original mass. Although initially hot, this white dwarf will slowly cool down over time, eventually becoming the background temperature of the Universe.

And that’s star evolution, from cloud of gas to white dwarf.

We have written many articles about stars here on Universe Today. Here’s an article about a supercomputer simulating star evolution, and here’s an article that explains what happens to the Earth 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?

Reference:
NASA: The Life Cycles of Stars

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When a star is still in the earliest stages of formation, it doesn’t have enough temperature in its core to ignite fusion of hydrogen and helium. Instead, the star shines with just the gravitational energy of its continue collapse. Astronomers call this pre-star a T Tauri star. This early stage lasts about 100 million years before nuclear fusion kicks in and it becomes a true star.

As you might know, stars start out as vast clouds of cold molecular hydrogen. Some event, like a nearby supernova, causes the cloud to collapse. As it collapses, the cloud fragments into separate pieces, each of which will eventually become a star. The first stage in a star’s life is as a protostar. This is where fresh material is still falling into the center of the collapsing cloud. After about 100,000 years or so, everything is collected, and the T Tauri star gets going.

T Tauri stars actually look quite similar to main sequence stars. Their surface temperatures are about the same as a star of a similar mass, but they’re more luminous because they have a larger diameter. But T Tauri stars get all their energy from the gravitational collapse of the material. They’re violent babies. There’s evidence that T Tauri stars are covered with active sunspots, and they produce extremely powerful stellar winds. Some of the brightness we see from here on Earth is the T Tauri star’s powerful stellar winds heating up a protoplanetary disk surrounding them.

Over 100 million years, these stars slowly collapse until the temperature and pressure at their core is sufficient to ignite stellar fusion. At this point, the star is converting hydrogen into helium at it’s core, and has become a main sequence star.

A star like our Sun will remain a main sequence star for about 12 billion years, as long as the fuel lasts in its core.

We have written many articles about stars here on Universe Today. Here’s an article about about two telescopes acting together to image a T Tauri star, and here’s an article about a T Tauri binary ejected from its system.

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?

Reference:
Wikipedia

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There are hundreds of billions of stars in the Milky Way alone; young and old, large and small, quiet and violent. But they all started out in the same way. Let’s take a look at thebirth of stars.

When we look out in the Milky Way, what we see are the stars, but a large part of the galaxy’s mass comes from clouds of molecular hydrogen; the stuff of future stars. These clouds are happy to drift along in the Milky Way for millions and even billions of years until some kind of event causes the cloud to collapse. It could be the collision between two clouds, or the shockwave of a passing supernova. This pushes the cloud over the top and gives gravity a chance to take over, and begin collapsing the cloud together.

As the cloud collapses, big pieces shear off. Each of these will become a star of their own. The mutual gravity on each chunk of the cloud continues to pull the material inward. The conservation of momentum from all the individual particles in the cloud makes it start to spin.

The first stage in the birth of a star is called a protostar. This is where the majority of the stellar material has collected together in ball in the center, but there is a huge disk of gas and dust obscuring it from our view. As long as there is still inflowing material, the object is a protostar. After enough material falls in on the star, jets of material blast out from either pole, announcing the new protostar to the Universe. The protostar stage takes about 100,000 years to complete.

Once there’s no more material falling inward, all that’s left is a hot ball of gas. Astronomers call this stage a T Tauri star. It doesn’t have internal temperature and pressure to begin nuclear fusion at its center, but it’s still a very hot object, and can appear as bright as a regular star. Over the next 100 million years, gravity continues to collapse the T Tauri star until the temperature at its core reaches the point that nuclear fusion can begin.

At this point, the star makes a transition to the main sequence stage of its life. This is a place it’ll remain for millions, billions, and even trillions of years depending on its mass.

We have written many articles about stars here on Universe Today. Here’s an article about the birth of the biggest stars, and some extreme starbirth in merging galaxies.

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?

Reference:
http://abyss.uoregon.edu/~js/ast222/lectures/lec11.html
http://burro.astr.cwru.edu/stu/advanced/stars_birth.html

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Did you ever wonder what stars are made of? You might not be surprised to know that stars are made of the same stuff as the rest of the Universe: 73% hydrogen, 25% helium, and the last 2% is all the other elements. That’s it. Except for a few differences here and there, stars are made of pretty much the same stuff.

After the Big Bang, 13.7 billion years ago, the entire Universe was a hot dense sphere. The conditions inside this young Universe were so hot that it was equivalent to being inside the core of a star. In other words, the entire Universe was like a star. And for the brief time that the Universe was in this state, nuclear fusion reactions converted hydrogen into helium to the ratios we see today.

The Universe kept expanding and cooling down, and eventually the hydrogen and helium cooled down to the point that it could actually start collecting together with its mutual gravity. This is how the first stars were born. And just like the stars we have today, they were made up of roughly 73% hydrogen and 25% helium. These first stars were enormous and probably detonated as supernovae within a million years of forming. In their life, and in their death, these first stars created some of the heavier elements that we have here on Earth, like oxygen, carbon, gold and uranium.

Stars have been forming since the Universe began. In fact, astronomers calculate that 5 new stars form in the Milky Way every year. Some have more of the heavier elements left over from previous stars; these are metal-rich stars. Others have less of these elements; the metal-poor stars. But even so, the ratio of elements is still roughly the same. Our own Sun is an example of a metal rich star, with a higher than average amount of heavier elements inside it. And yet, the Sun’s ratios are very similar: 71% hydrogen, 27.1% helium, and then the rest as heavier elements, like oxygen, carbon, nitrogen, etc. Of course, the Sun has been converting hydrogen into helium in its core for 4.5 billion years.

Stars everywhere are made of the same stuff: 3/4 hydrogen and 1/4 helium. It’s the stuff left over from the formation of the Universe, and one of the most elegant pieces of evidence to help explain how we’re here today.

We have written many articles about stars here on Universe Today. Here’s an article about how metallic stars can yield planets, and some identical stars that might not be so identical.

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
Bluffton University