On a clear dark night, a few thousand stars are visible to the naked eye. With binoculars and powerful telescopes, we can see so many stars that we could never hope to count them.
Even though each individual star is unique, all stars share much in common. The Sun, which is the source of virtually all light, heat and energy reaching the Earth, is the nearest star. Today, we know that stars are born from interstellar gas clouds, shine by nuclear fusion and then die, sometimes in dramatic ways.
Birth of Stars
Stars are born in cold interstellar clouds like the Orion Nebula and the Eagle Nebula. In these stellar nurseries, dense regions undergo gravitational collapse to form a rotating gas globule. As the globule collapses, the temperature and pressure increase and it spins faster. This causes the globule to have a central core and a surrounding flattened disk of dust. The central core becomes a star, while the disk may coalesce into planets and asteroids. The process of collapse takes between 10,000 and 1,000,000 years.
Lives in the Balance
A star's life is an extended battle between two opposing forces: gravity and pressure. A star can maintain its internal pressure only if it continually generates energy to replace the energy that it radiates into space. This energy comes primarily from nuclear fusion of light elements into heavier elements, through which a star shines for millions or billions of years.
All stars spend a significant amount of their lifetime fusing hydrogen to helium. This phase of the star's life is called the main sequence. Examples of main sequence stars are the Sun, Vega, Sirius and Spica. When the hydrogen in the core of the star is depleted, the envelope of the star expands tremendously and the star becomes a red giant. Examples of red giants are Betelguese, Arcturus, Aldebaran and Antares.
As a star has a limited amount of material in its core, it cannot rely on thermal energy to resist gravity forever and its ultimate fate depends on whether something other than thremal pressure manages to halt the unceasing crush of gravity.
Death of Stars
The outcome of a star's struggle between gravity and pressure depends entirely on its birth mass. Stars with masses below about 5 solar masses swell into red giants near the ends of their lives, after which the envelope is ejected as a planetary nebula, while the core becomes a white dwarf. Examples of planetary nebulae are the Ring Nebula, Eskimo Nebula, Helix Nebula and the Cat's Eye Nebula.
Most stars are believed to have their origin in clusters. There are two kinds of star clusters: open clusters and globular clusters.
Open clusters are physically related groups of stars held together by mutual gravitational attraction. Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, and are found along the band of the Milky Way in the sky. Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster due to velocity changes from tidal interactions with other objects. Examples of open clusters are M37 and M52.
Globular clusters are gravitational bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to 200 light years in diameter. They are believed to be very old, estimates of their ages being 12 to 20 billion years. Examples of globular clusters are M13 and M28.
- What is a star's "spectrum"? (Beginner)
- Can I buy a star? (Beginner)
- Why aren't there any green stars? (Intermediate)
- How does the color index of a star relate to its actual color? (Intermediate)
- Why are stars and planets round? (Beginner)
- Are all stars the same? (Beginner)
- What is a nova? (Beginner)
- Can one star orbit another? (Beginner)
- How many moles of stars are in the known universe? (Beginner)
- What is the largest star? (Beginner)
- Are there stars outside of galaxies? (Beginner)
- Why are there stars? (Beginner)
- What is the difference between a "star" and a "sun"? (Beginner)
- Is it possible to count the stars? (Beginner)
- Is there a list of the sizes of different kinds of stars? (Beginner)
- How many stars are there in our Galaxy (Milky Way)? (Intermediate)
- What is a Cepheid variable? (Intermediate)
- What kinds of star clusters are there? (Intermediate)
- How did the stars get named? (Intermediate)
- How does a star take mass from another star? (Intermediate)
- What would happen if two stars collided? (Intermediate)
- How do stars move in the Galaxy? (Intermediate)
- What are "blue stragglers" in globular clusters? (Intermediate)
- Does the particles involved in a star's final core collapse travel faster than the speed of light? (Intermediate)
- Can you tell me the characteristics of W, R, N and S type stars? (Advanced)
- Why aren't accretion disks around giant stars as hot as accretion disks around black holes? (Advanced)
- Why is pressure broadening greater in low mass stars than in high mass stars? (Advanced)
- How do we measure distances to other stars? (Beginner)
- How can I measure the distance of a star? (Beginner)
- What are the closest and/or brightest stars? (Beginner)
- How can we measure distances to more stars? (Intermediate)
- Why don't astronomers use everyday units to measure distances (what is an AU or a pc)? (Intermediate)
- How is the Cepheid yardstick validated? (Advanced)
- How can I calculate distances between stars? (Advanced)
- What do stellar classifications mean? (Beginner)
- How do you measure the mass of a star? (Beginner)
- How does the brightness of a star depend on its distance from us? (Beginner)
- How do astronomers measure the brightness of stars? (Intermediate)
- Do the magnitudes and colors of stars ever change? (Intermediate)
- What can we learn from the color of a star? (Intermediate)
- How can we distinguish a star's "real" color from the change in color that we observe due to the star's motion? (Intermediate)
- What is apparent magnitude? (Intermediate)
- How do astronomers measure the radius of a star? (Intermediate)
- How do we know what other planets (and stars, galaxies, etc.) are made of? (Intermediate)
- How do we find out about the star formation histories of galaxies? (Intermediate)
- Where are stars born? (Beginner)
- How many stars are born and die each day? (Beginner)
- Has anyone ever seen a star "turn on"? (Beginner)
- What is the life cycle of a star? (Intermediate)
- Do binary or triple star systems form together? (Intermediate)
- Why does Barnard 68 appear dark? (Intermediate)
- Why are stars of different masses? (Advanced)
- Could we be seeing dead stars? (Beginner)
- Do dead stars still shine? (Beginner)
- Can we see dead stars with the naked eye? (Beginner)
- What happens when you change the mass of a White Dwarf or Neutron Star? (Intermediate)
- Are planetary nebulae the result of supernovae? (Intermediate)
- What process would bring about a quark star? (Intermediate)
- If a white dwarf is a "dead" star, why is it so hot? (Intermediate)
- Where are the protons and electrons in a Neutron Star? (Intermediate)
- Why don't all neutron stars become pulsars? (Advanced)
- What would happen if there was a collision between two white dwarfs? (Advanced)
- Why can't you see stars during the day? (Beginner)
- Why do stars twinkle? (Beginner)
- Why do stars change colour when they twinkle? (Beginner)
- What is the observational difference between a star and a planet? (Beginner)
- Is there a "South Star"? (Intermediate)
- Why is it easier to see a star if you look slightly to the side? (Intermediate)
- What are the boundaries of the constellations? (Beginner)
- Which constellation does our Sun belong to? (Beginner)
- Will winter and summer constellations change due to abnormal warmth or cold on Earth? (Beginner)
- How many constellations are there? (Beginner)
- What are the names of the three stars in Orion's belt? (Beginner)
- Do constellations looks the same from space? (Beginner)
- Why do constellations look the same after several years even though all the stars are moving? (Intermediate)
- What are constellations used for? (Intermediate)
- How long does it take the Big Dipper to move in the sky? (Intermediate)
- What month was it when Odysseus saw Orion in 1000BC? (Advanced)
The Ask an Astronomer team's favorite links about Stars:
- Stars: This website gives you a lot of information about individual stars. Look here if you want details (like age, color, distance etc.) of prominent stars.
- Constellations and their Stars: This site tells you about constellations and and gives information about the stars associated with them. It also has interactive sky charts and information about Messier objects.
- The Hipparcos Astrometry Mission: Hipparcos is a satellite which measured the distances to thousands of stars through stellar parallax. This website has links to the Hipparcos catalog and other links related to Hipparcos.
- The HR diagram: This site gives a schematic HR diagram showing evolutionary tracks of stars of different masses.
- 3-D Star Chart Viewer: This website has links to a downloadable 3-D Star Chart Viewer for Linux.
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