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About Stars

How do stars form?
Stars form within cold, slowly rotating, interstellar clouds of hydrogen and helium gases and dust, called molecular clouds, that contain huge masses of material – enough to form several stars. Individual stars form in regions of these clouds, called cores, at different times. A protostar forms when the great mass contained within a core collapses from its own gravity, forming a huge spinning ball of gas. As the ball continues to collapse, it heats up to a temperature where fusion occurs (atoms are fused together, forming different atoms and releasing energy) and a star is born.

The mass of the core determines the mass of the star. The more massive the core, the more massive the star — and the faster it forms. The mass of the star controls its evolution. Some protostars do not have enough mass to initiate fusion. Astronomers call these “failed” stars “brown dwarfs.”

Often new stars are not seen because there is so much gas and dust hiding them. Hints of their existence can be found where they make the surrounding cloud glow. They also heat the dust and they can be “seen” with telescopes that collect infrared (heat) images. Eventually the surrounding dust and gas is either collected into the new star, or the star releases a powerful stream of energy — stellar wind — that blows the dust and gas away into space.

Hubble Space Telescope images of the Eagle Nebula


Hubble Space Telescope images of the Eagle Nebula, located about 7000 light-years from Earth in the constellation Serpens. The nebula is made of interstellar gas and dust and is a region of star formation.

Image courtesy of Jeff Hester and Paul Scowen of Arizona State University and NASA.

Images from a portion of the Orion Nebula, taken by the Hubble Space Telescope.


Images from a portion of the Orion Nebula, taken by the Hubble Space Telescope. This cloud of dust and gas is a place of star formation.

Image courtesy of NASA and C. R. O'Dell and S. K. Wong (Rice University).


Hubble Space Telescope image of a giant galactic nebula (NGC 3603).


Hubble Space Telescope image of a giant galactic nebula (NGC 3603). The huge pillars of gas and dust are the sites of star birth. Stars in the center of the image are older stars near the ends of their life cycles.

Image courtesy of Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (University of Washington), You-Hua Chu (University of Illinois Urbana-Champaign), and NASA.

Is our star special?
Of course! It is the center of our solar system and very important to us. Our star provides us with light and heat. The heat warms us and powers the movements of our atmosphere and ocean. The light is used by plants to make food for us and to put oxygen in our atmosphere. In other ways, however, our star is “average.” It is a medium-sized star of average brightness. Our Sun is considered “stable” — a main-sequence star. It is in the stage of its life where it is fusing hydrogen nuclei into helium and giving off energy.

You’re a star! Every element (except hydrogen) ultimately has been made by fusion inside stars. These materials are combined and recombined into new stars in the galaxies across our universe, terrestrial and gas planets, and all the living things on Earth.
Do stars change with time?
Yes! Stars evolve. Just think of the energy given off by our own Sun. Nuclear reactions allow our Sun — and all the other stars — to continue to give off energy. However, making this energy consumes the fuel of the Sun. Eventually, it will run out of fuel. But do not worry, our Sun will keep shining as it is for about another 3 to 5 billion years.

The destiny of a star depends on its mass. Stars spend most of their lives as main-sequence stars — stars that are fusing hydrogen fuel into helium. The more massive the star, the faster it transforms hydrogen into helium, using up its fuel. The smaller the star, the slower the fusion, and the longer it takes to use all the fuel.

When a star about the mass of our Sun uses all the fuel in its interior, the star core collapses. This heats up the outer layers. The hydrogen in the outer layers starts to burn very quickly, further heating the star and causing it to expand. As the star increases in size, its surface temperature decreases (because of the larger surface area) and the star color changes to a deep red. The star in this stage of evolution is called a “red giant.”

Inside the star’s collapsed interior, the temperatures increase until they reach the point where helium begins to burn. As the helium is used, it in turn produces other elements. Gradually the material deep in the star is converted into material that will not burn because it requires temperatures hotter than the dying star can produce.

As the star is burning the last of its fuel, it may go through several pulses where it expands and contracts. In these pulses, it may expel material from the outer layers, creating a cloud of material surrounding a small star. This cloud is called a “planetary nebula” and the star is a “white dwarf” — a hot white star that is about the size of Earth. While it is hot, it is not hot enough to burn fuel, and the white dwarf eventually c ools into a black dwarf. This is the future of our own Sun.

Scientists are not sure what happens to stars that are less massive than our Sun. If they are much less massive, they never really become stars in the first place — they become brown dwarfs. But those that do reach temperatures where fusion can occur burn the fuel so slowly that they have not evolved from the main sequence yet, so scientists have no examples of small stars near the end of their lives.

Stars that are more massive than our Sun — say ten or more times more — ultimately blow apart in a supernova! These stars go through a process similar to stars the size of our Sun, but they swell into red supergiants that can be more than 950 million kilometers (about 600 million miles) across!. However, as their massive centers collapse under immense gravity, the components of the atoms rearrange and recombine, releasing energy to blow apart the outer layers in a huge explosion. These supernovas disperse elements that will be incorporated into molecular clouds and future stars.

What is left behind is a neutron star — a small superdense star. Neutron stars may be as little as 15 kilometers (9 miles) across and they spin very quickly on their axes. The most massive stars, those 15 times more massive than our Sun, collapse into a single point, called a black hole. Gravity is so strong that even light cannot escape. Some scientists believe that black holes contain all the socks that have gone missing in dryers.

Hubble Space Telescope image of the remnants of a supernova (1987A).


Hubble Space Telescope image of the remnants of a supernova (1987A) in the Large Magellanic Cloud. Supernovas are the catastrophic destruction of a massive star. The remnants can be seen in the center of this image as the red ring-like features, surrounded by gas and dust.

Image courtesy of the Hubble Heritage Team (AURA, Space Telescope Science Institute, NASA.


Hubble Space Telescope image of the Crab Nebula.


Hubble Space Telescope image of the Crab Nebula.


Hubble Space Telescope image of the Cat’s Eye Nebula, NGC 6543.


Hubble Space Telescope image of the Cat’s Eye Nebula, NGC 6543. This complex nebula has multiple rings of gas and may be a double-star system. The bright point of light at the center marks the location of the twin stars. Scientists estimate that the nebula is about 1000 years old.

Image courtesy of NASA and J. P. Harrington and K. J. Borkowski (University of Maryland).



How are stars classified?
Stars are classified by their mass, the color of visible light they give off, and their temperatures. There are general relationships between these characteristics. Low-mass stars – like our Sun – have masses that range from 0.1 to 4 times that of our Sun. These stars tend toward the cool side; they “shine” yellow to white for most of their lives while they are fusing hydrogen to helium.  Their surface temperatures are about 5000-6000 K. Middle- to high-mass stars have masses at least 4 times greater than our Sun. They tend to “shine” bluish white during most of their lives, and have surface temperatures greater than 7500 K – some reach up to 30,000 K! When stars approach the ends of their lives, as they run out of fuel, they expand into red giants or red supergiants. Because of their greater surface area, these stars have cooler surfaces with temperatures less than 5000 K.

You can see stars of different temperatures — marked by different colors — when you find Orion in the night sky. There is a bright red star in the upper left corner — Betelgeuse. In the bottom right corner is Rigel, a blue star. Binoculars may help you distinguish the colors.

Star birth is happening now! Scientists estimate that ten new stars are born every year in our Milky Way galaxy.
Why do stars twinkle? Stars twinkle because their light passes though Earth’s thick, turbulent atmosphere to Earth’s surface, where we view them. As the star light passes through the layers of the atmosphere, it gets bent (or refracted) a little whenever it encounters air pockets of different density (hot or cold). The bending makes the star’s position appear to “jump” or twinkle. Light coming from stars closer to our horizon twinkle more because the light is passing through a thicker portion of atmosphere than star light coming from overhead. If you viewed stars from above Earth’s atmosphere, say, sitting aboard the Hubble Space Telescope, they would not twinkle at all.