Article

The Stars

  • Grades: 6–8, 9–12

The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

How does a star form?
There are several steps in the formation of a star:

First the cloud of gas and dust comes together, due to gravity, to form a "protostar" (a hot blob that is not quite a star but will eventually become a star) that takes thousands of years.

Then the center of the blob becomes hot enough to give off visible light and most of the cloud of gas and dust has fallen into it. So now it looks like a star. That takes about a million years, if the star is about the mass (weight) of our sun.

Then the rest of the cloud of gas and dust either falls into the star or gets blown away, and the star gets hotter and smaller due to gravity. Finally the center becomes so hot that the hydrogen gas starts to undergo nuclear reactions to become helium, which provides the energy for the star to keep shining for billions of years. It takes about 20 million years for the new star to get to this point. The funny thing is that a baby star is BIGGER than an adult star!

What are the biggest stars?
The biggest stars are known as "red supergiants." The star Betelgeuse (which is in the constellation Orion) is one. If you plopped Betelgeuse into the middle of our solar system, it would fill it out to roughly the orbit of Jupiter! Red supergiants are about 400 times larger than our sun. That would be about 300 million miles across, which is over three times the distance between Earth and the sun. If the sun were a red supergiant, it would swallow up Mercury, Venus, Earth, Mars, and some asteroids!

How many times hotter, brighter, and larger is Betelgeuse than the sun?
Betelgeuse is actually cooler than our sun. The sun's surface temperature is about 5,800° Kelvin (about 10,000° Fahrenheit), and Betelgeuse is roughly half that, about 3,000° Kelvin (about 5,000° Fahrenheit). That is why it is red — red stars are cooler than the sun, blue-white stars are hotter.

Betelgeuse is, however, much bigger and brighter. It is about 500 times bigger than our sun. If you put Betelgeuse into our solar system, it would swallow up Mercury, Venus, Earth, and Mars!!! Also it is about 10,000 times brighter than our sun (because a larger star is brighter).

How many stars are in space?
We think that there are about 200 billion stars in our galaxy, the Milky Way. There are billions of other galaxies, too. So the total number of stars in space is HUGE!!!! Of course you can't see them all. Most of them are too faint to see except through a big telescope.

How come some nights we see stars where there were none before?
I'm not sure why you see stars on some nights where there were none before. Sometimes the sky is very clear and you can see fainter stars, while other times it is a little hazy and you can see only the brighter stars.

How do stars twinkle?
Scintillation, or the "twinkling" we see of stars in the sky, is due to motions in the earth's atmosphere. I have watched this many times through a telescope! So it is very much connected to our atmosphere and its weather.

When we look at a star from the surface of the earth, we are also looking through the various layers of the atmosphere. Air has several kinds of motions. There is the wind, of course, but it also has a convective (bubbling) motion where blobs of hot air rise, cool, then fall, to be warmed by the warm earth below. The astronomers and engineers who build telescopes are very familiar with this, because some of these telescopes are designed to get around the blurring effects of these motions.

When I used to observe at Perkins Observatory in Ohio, I noticed that the scintillation (we also call it "seeing") changed in a predictable pattern depending on the weather. Right after a front passed, the atmosphere was turbulent (bubbling a lot) and the image of the star was large and blobby. The next night the air was calmer, and the image of the star looked smaller and more stable. This would continue, until the cirrus clouds that come before the next front arrived. Then the image was the smallest and most stable (the icy cirrus clouds form in very calm air).

Why are stars so bright?
I think of stars being rather faint because they are so far away! Most stars are very similar to our sun. In fact the sun is a pretty normal kind of star. It's much brighter than the other stars because it is close by. Even the closest star (other than the sun) is very far away. To give you an idea of how far, we can compare the time that it takes for light to travel from one place to another. Light is very, very fast; it travels 186,000 miles in one second.

Even so it takes about eight minutes for light to travel from the sun to the earth. How long does it take for light from the sun to travel to the nearest star? Over four YEARS!

When you look at the stars at night, some are closer and some are farther. Most of the stars that are the brightest are also the closest to us. The farther away the star is, the fainter it is.

Is it true that a star is a burning ball of fire?
Well, no, stars are not on fire although they look that way. We sometimes talk about them "burning," which can be confusing because we don't mean burning as in fire. Stars shine because they are extremely hot (which is why fire gives off light — because it is hot). The source of their energy is nuclear reactions going on deep inside the stars. In most stars, like our sun, hydrogen is being converted into helium, a process which gives off energy that heats the star. The inside is actually millions of degrees, extremely hot! That warms the outer layers of the star, which gives off heat and light.

Something that is on fire, like the wood in a fireplace, requires oxygen to burn. The temperature of such a fire is hot, but not as hot as a star!

What do the stars look like up close?
Did you know that our sun is a star? It's a pretty ordinary, normal kind of star. So that's what a star looks like up close. Some stars are bigger, some are smaller, some are hotter (and look bluish-white) and some are cooler (and may look yellow, orange, or red).

What is a baby star like?
Baby stars are born in big, dark clouds of gas and dust. They start out all wrapped up in these clouds, like blankets protecting them. But there is one thing about baby stars you might not expect. They start out BIG and get smaller as they grow older! That is because the baby stars are formed out of those clouds, and gravity pulls them together to make a star. The baby star starts out big and cool, surrounded by clouds, so you can't see it. But as it gets older, it gets hotter and brighter. The clouds are blown away and then you can see the baby star (now more like a "toddler").

Where do "young" stars get their energy from if nuclear fusion has not yet taken place? When does it finally take place?
Young stars get their energy from gravity. They are slowly contracting, and as they squeeze together that generates energy which gets radiated away as light. Once the center of the star is hot and dense enough (millions of degrees!), nuclear fusion can start. It takes a star the size of our sun about 20 million years to reach this point. Once it starts nuclear fusion, the star can shine for about 10 billion years.

How can you tell how old a star is?
It's not easy getting the age for a star. Here are two methods we use:

The first method is to look at the star's spectrum (formed when we spread out the light from the star into the various colors, like a rainbow). Using special instruments, we can find dark lines in the spectrum that correspond to the elements in a star. The element lithium can be used to get an age for a star because the amount of lithium in a star decreases with time. This is because it gets converted to other elements by nuclear reactions. So if we can measure the amount of lithium in the star, we can get its age (the less lithium, the older the star).

The second method is to find the age of a cluster, or group, of stars. Many stars form together in clusters, so they all have the same age. We know from our calculations that very big, massive stars burn up their nuclear fuel very fast and have short lifetimes, while smaller stars use up their fuel much more slowly and can continue giving off light for much longer. By looking at the various stars in the cluster we can see which ones have used up their fuel (and become red giants) and which ones are still shining as usual. Then we can figure out that all the stars in the cluster must be the same age as the age of the stars that just recently used up their nuclear fuels. For instance, if all the stars that are greater than three times as massive as our sun have run out of nuclear fuel and have become red giants, then we know that all the stars in the cluster are 500 million years old.

Why are some stars brighter than the sun?
Well of course here on Earth we see the sun brighter than anything else! That is because the sun is so much closer than the other stars.

But if you could line up a bunch of stars, including our sun, all at the same distance, you would see that some stars are brighter and some are fainter than our sun. The biggest, heaviest stars can make more energy and shine more brightly than the sun. The smaller, lighter stars make less energy and shine less brightly than our sun. So it all depends on how big and heavy a star is.

How are stars in a nebula formed?
It's all due to gravity. The nebula consists of gases, mostly hydrogen, and also dust. The dust is just what you would expect, tiny rocky particles. If the nebula is cold and dark, denser blobs can form in it. Those blobs have gravity and can put surrounding gas and dust into them. As they get bigger, they have stronger gravity and can pull more and more gas and dust to them.

The inner layers of gas and dust start to warm from the pressure of the gas and dust above. The dust is vaporized and turned into gas. When the inner gases get hot enough, the blob — now a protostar, or very young star — begins to glow. At first it can be seen only in infrared light, but as it warms up and brightens it can be seen as a red star. Now the star's heat and light blows away the surrounding gas and dust, and the new star can finally be seen in the nebula.

Can a nebula form two stars at a time?
It sure can. In fact over half the stars in the sky are actually binary (or triple) stars, in which two (or three) stars formed together and are in orbit around each other. In addition, stars tend to form in large groups within huge dark clouds of gas and dust. There can be hundreds of stars forming in these big dark clouds.

How did stars get the name "stars"?
People have seen the stars overhead for thousands of years. Even though they didn't know what they were (or had some odd ideas about that!), they had a name for them. The ancient Greeks said "aster" (from which we get the word astronomy), while the Romans said "stella." Our word star comes from the Old German word for star, which was sterno (the modern German word for star is stern).

Why are stars given names?
We give stars names so we can talk about them with each other. This is just like giving people names, so you can call your classmate "John" instead of "red-haired boy with freckles wearing blue jeans."

The brightest stars have names that were originally Arabic in origin. For instance, the star Betelgeuse's name means "armpit" (it is in the constellation of Orion, the hunter, at his armpit).

Stars are listed in catalogs that give information like the position (in coordinates similar to latitude and longitude on the earth), the brightness of the star, its color, and so on. For instance, we often use the Henry Draper catalog designation such as HD 7762 (for star number 7762 in that catalog). There are lots of catalogs with all kinds of names. Some stars are listed in more than one catalog, so they have more than one name.

How did they come up with the names for the constellations?
In ancient times, people looked up at the stars and picked out patterns that they saw. They often associated these patterns with pictures from the stories that they told. Most of the constellations that we use today come from the ancient Romans, and they depict many of the people and animals from their myths.

Why did they choose to call them constellations?
The word constellation means "with" (con) "stars" (stella), and comes from the Latin word constellatio.

How fast does a shooting star go?
As you probably know, a "shooting star" is actually a meteor, a tiny bit of rock in space. Meteors are moving very fast, which is why they burn up when they hit our earth's atmosphere and make a nice, bright "shooting star." Once meteors hit Earth's atmosphere they are traveleling at least 25 miles per hour. But some go as fast as 160,000 miles an hour!

What instruments do you use to study these stars?
The instrument I have used the most to study baby stars is an astronomy satellite called the International Ultraviolet Explorer spacecraft. I studied the ultraviolet light from my baby stars with it for many years to try to understand how they behave. Ultraviolet light is absorbed by the earth's atmosphere, so the only way to measure it is by using a satellite.

How exactly do astronomers use spectroscopes and what do they tell about stars?
Spectroscopes are a very important tool used by astronomers. As you probably realize, astronomers must rely on the light that we can measure from the various astronomical objects. We can't put a star into a laboratory!

The spectrum of a star can tell us the temperature, size, and composition of the star. It can also tell us how fast it is moving. If there are two stars in orbit around each other, a series of measurements can be used to get their masses (weights). We can tell if the star has strong magnetic fields. Sometimes we can get the age of the star. Most of what we know about stars has been learned from their spectra!

How many colors of stars are there and what do they mean?
Stars come in colors from red, orange, yellow, white, bluish-white, and blue. The color depends on how hot the star is. A red star is the coolest, but still is about 5,000° Fahrenheit! Our sun is yellowish-white and the surface is about 10,000° Fahrenheit. The hottest stars are blue, and can be as hot as 200,000,000° Fahrenheit at their core!

Could a star connect to another star?
Yes, sometimes it happens. For instance, two stars may start out as a pair in orbit around each other. Then the heavier star (which ages faster) may become a red giant star, expanding big enough that the outer layers are close to the second star. Then some of the gas in the outer parts of the red giant may get pulled by the second star's gravity and get pulled onto the second star. If the red giant expands large enough and the second star is close enough, it could even end up inside the red giant star!

What is a brown dwarf star?
A brown dwarf is a very small star, so small that it can't produce energy through nuclear reactions the way the normal stars do. It glows mostly in infrared light (I guess that's where they came up with the "brown" part, actually it would look deep red to us) and is not as bright as other stars. During its long lifetime, it slowly contracts, gives off infrared light, and gets dimmer and dimmer.

How long does it take a white dwarf star to change to a black dwarf star?
It takes a long time — several billion years — for a white dwarf star to completely cool and become a "black dwarf."

If a person mapping the earth is a cartographer, is there a specific name for a person who draws constellations?
A cartographer makes maps, and I think it doesn't matter what the map is of. So they can make maps of the earth, the moon, Mars, and the constellations too.

What is the North Star?
The North Star is known as Polaris, or alpha Ursae Minoris (the brightest star in the constellation Ursa Minor, the "Little Bear," also known as the "Little Dipper"). It is the closest bright star to the direction of the earth's North Pole, although it isn't exactly at the North Pole. You may know that the direction of the earth's North Pole changes with time, as the earth very slowly wobbles in a circle every 26,000 years. Thousands of years ago, other stars were near the North Pole instead of Polaris!

Polaris is a yellow supergiant star. It is a little hotter than our sun, and much bigger and brighter. It is also a star near the end of its life. In fact it has a little variable in its brightness, because it is a little unstable (so it pulses, but it won't explode). It varies in brightness by about ten percent every four days. It is about 430 light-years away.

What are some legends about the North Star?
For many hundreds of years, Polaris has been well known as a guide pointing to the North Pole. It has had various names along these lines — the Lodestar, the Steering Star, the Pivot Star, and the Ship Star. The North Star has often been used as a symbol for constancy and faithfulness. In ancient times, it was thought to lie at the point around which the earth spins — as if there were actually a spindle through the earth that stuck into the sky. The Chinese thought that the star was at the top of the heavenly Mountain of the World at the North Pole. In India, the great temples depict the Cosmic Mountain.

But here's a funny thing. In ancient times, the star we call the North Star was NOT the star closest to the North Pole. Beta Ursae Minoris was (the second brightest star), and in those times people called THAT star the North Star instead of Polaris.

  • Subjects:
    Astronomy and Space, Universe and Stars, Real-World Science, Science through Literature
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