Understanding Stars: Characteristics and Classification

1. Chapter 26 The Universe

2. Stars A star is a large, glowing ball of gas in space, which generates energy through nuclear fusion in its core. The closest star to Earth is the sun, which is considered to be a fairly average star.

3. Stars Parallax Stars are so far away that astronomers cannot measure their distances directly. Astronomers are able to observe stars from two different positions–opposite sides of Earth’s orbit. Nearby stars appear to move against the more-distant background stars.

4. Stars • The apparent change in position of an object with respect to a distant background is called parallax. • Astronomers measure the parallax of nearby stars to determine their distance from Earth.

5. Stars With the invention of the telescope, astronomers could measure the positions of stars with much greater accuracy. • The closer a star is to Earth, the greater is its parallax. • Astronomers have measured the parallax of many nearby stars and determined their distances from Earth.

6. Stars The Light-Year Because stars are so far apart, it’s not practical to measure their distances in units that might be used on Earth, such as kilometers. • A light-year is the distance that light travels in a vacuum in a year, which is about 9.5 trillion kilometers. • Proxima Centauri, the closest star to the sun, is about 4.3 light-years away.

7. Stars Astronomers classify stars by their color, size, and brightness. Other important properties of stars include their chemical composition and mass.

8. Stars Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up 96 to 99.9 percent of the star’s mass.

9. Stars Color and Temperature A star’s color indicates the temperature of its surface. • The hottest stars, with surface temperatures above 30,000 K, appear blue. • The surfaces (photospheres) of relatively cool red stars are still a toasty 3000 K or so. • Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun.

10. Stars Brightness Astronomers have discovered that the brightness of stars can vary by a factor of more than a billion. Stars that look bright may actually be farther away than stars that appear dim.

11. Stars The sun appears very bright to us because it is much closer than other stars. The brightness of a star as it appears from Earth is called its apparent brightness. The apparent brightness of a star decreases as its distance from you increases.

12. Stars Absolute brightness is how bright a star really is. A star’s absolute brightness is a characteristic of the star and does not depend on how far it is from Earth. You can calculate a star’s absolute brightness if you know its distance from Earth and its apparent brightness.

13. Stars Size and Mass Once astronomers know a star’s temperature and absolute brightness, they can estimate its diameter and then calculate its volume. The masses of many stars can be determined by observing the gravitational interaction of stars that occur in pairs. For most stars, there is a relationship between mass and absolute brightness.

14. Stars Composition A spectrograph is an instrument that spreads light from a hot glowing object into a spectrum. Astronomers can use spectrographs to identify the various elements in a star’s atmosphere.

15. Stars This is the spectrum of a star. The dark absorption lines indicate the presence of various elements in the star.

16. Stars H-R diagrams are used to estimate the sizes of stars and their distances, and to infer how stars change over time.

17. Stars Stars can be classified by locating them on a graph showing two easily determined characteristics. Such a graph is called a Hertzsprung-Russell diagram, or H-R diagram. An H-R diagram is a graph of the surface temperature, or color, and absolute brightness of a sample of stars.

18. Stars The horizontal axis shows the surface temperatures of stars. A star’s color is directly related to its surface temperature. The hottest blue stars are on the left and the coolest red stars are on the right. Surface temperatures of stars range from less than 3000 K to more than 30,000 K.

19. Stars The vertical axis of the H-R diagram shows absolute brightness, with the brightest stars at the top and the faintest at the bottom. The absolute brightnesses of stars vary even more than temperature, ranging from about one ten-thousandth to a million times that of the sun.

20. Stars • A star’s placement on an H-R diagram indicates its absolute brightness and surface temperature (or color).

21. Stars Main-Sequence Stars Stars occur only in certain places on the H-R diagram. Most stars are found along a diagonal band running from the bright hot stars on the upper left to the dim cool stars on the lower right. Astronomers call this diagonal band on the H-R diagram the main sequence. About 90% of all stars are found on the main sequence. The sun lies near the middle of this band.

22. Stars Giants and Dwarfs In general, two factors determine a star’s absolute brightness: its size and its surface temperature. An H-R diagram shows a star’s absolute brightness and surface temperature. • If you compare two stars at the same temperature, the brighter one must be larger. • Hotter stars are brighter than cooler stars of the same size.

23. Stars The very bright stars at the upper right of the H-R diagram are called supergiants. Supergiants are much brighter than main-sequence stars of the same temperature, so they must be very large compared with main-sequence stars.

24. Stars Supergiants range in size from 100 to 1000 times the diameter of the sun. Just below the supergiants on the H-R diagram are the giants—large, bright stars that are smaller and fainter than supergiants

25. Stars Below the main sequence in the lower part of the H-R diagram are white dwarfs. • A white dwarf is the small, dense remains of a low- or medium-mass star. • White dwarfs are hot but dimmer than main-sequence stars of the same temperature.

26. Stars • The diameter of a red giant is typically 10–100 times that of the sun and more than 1000 times that of a white dwarf.

27. Life Cycle of Stars A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

28. Life Cycle of Stars A nebula is a large cloud of gas and dust spread out over a large volume of space. • Some nebulas are glowing clouds lit from within by bright stars. • Other nebulas are cold, dark clouds that block the light from more-distant stars beyond the nebulas. Stars form in the densest regions of nebulae. Gravity pulls a nebula’s dust and gas into a denser cloud. As the nebula contracts, it heats up.

29. Life Cycle of Stars A contracting cloud of gas and dust with enough mass to form a star is called a protostar. As a protostar contracts, its internal pressure and temperature continue to rise. Pressure from fusion supports the star against the tremendous inward pull of gravity.

30. Life Cycle of Stars A star’s mass determines the star’s place on the main sequence and how long it will stay there.

31. Life Cycle of Stars Stars spend about 90 percent of their lives on the main sequence. In all main-sequence stars, nuclear fusion converts hydrogen into helium at a stable rate. There is an equilibrium between the outward thermal pressure from fusion and gravity’s inward pull. The amount of gas and dust available when a star forms determines the mass of each young star.

32. Life Cycle of Stars The most massive stars have large cores and therefore produce the most energy. High-mass stars become the bluest and brightest main-sequence stars. These blue stars are about 300,000 times brighter than the sun. Because blue stars burn so brightly, they use up their fuel relatively quickly and last only a few million years.

33. Life Cycle of Stars Stars similar to the sun occupy the middle of the main sequence. A yellow star like the sun has a surface temperature of about 6000 K and will remain stable on the main sequence for about 10 billion years.

34. Life Cycle of Stars Small nebulas produce small, cool stars that are long-lived. A star can have a mass as low as a tenth of the sun’s mass. The gravitational force in such low-mass stars is just strong enough to create a small core where nuclear fusion takes place. This lower energy production results in red stars, the coolest of all visible stars. A red star, with a surface temperature of about 3500 K, may stay on the main sequence for more than 100 billion years.

35. Life Cycle of Stars The dwindling supply of fuel in a star’s core ultimately leads to the star’s death as a white dwarf, neutron star, or black hole.

36. Life Cycle of Stars When a star’s core begins to run out of hydrogen, gravity gains the upper hand over pressure, and the core starts to shrink. • The core temperature rises enough to cause the hydrogen in a shell outside the core to begin fusion. • The energy flowing outward increases, causing the outer regions of the star to expand. The expanding atmosphere moves farther from the hot core and cools to red. • The star becomes a red giant.

37. Life Cycle of Stars • The collapsing core grows hot enough for helium fusion to occur, producing carbon, oxygen, and heavier elements. • In helium fusion, the star stabilizes and its outer layers shrink and warm up. • The final stages of a star’s life depend on its mass.

38. Life Cycle of Stars Low- and Medium-Mass Stars Low-mass and medium-mass stars, which can be as much as eight times as massive as the sun, eventually turn into white dwarfs. • Stars remain in the giant stage until their hydrogen and helium supplies dwindle and there are no other elements to fuse. • The energy coming from the star’s interior decreases. • With less outward pressure, the star collapses.

39. Life Cycle of Stars • The dying star is surrounded by a glowing cloud of gas, called a planetary nebula. • As the dying star blows off much of its mass, only its hot core remains. • This dense core is a white dwarf. A white dwarf is about the same size as Earth but has about the same mass as the sun. • White dwarfs don’t undergo fusion, but glow faintly from leftover thermal energy.

40. Life Cycle of Stars High-Mass Stars The life cycle of high-mass stars is very different from the life cycle of lower-mass stars. • As high-mass stars evolve from hydrogen fusion to the fusion of other elements, they grow into brilliant supergiants, which create new elements, the heaviest being iron. • A high-mass star dies quickly because it consumes fuel very rapidly.

41. Life Cycle of Stars • As fusion slows in a high-mass star, pressure decreases. • Gravity eventually overcomes the lower pressure, leading to a dramatic collapse of the star’s outer layers. • This collapse produces a supernova, an explosion so violent that the dying star becomes more brilliant than an entire galaxy.

42. Life Cycle of Stars Supernovas produce enough energy to create elements heavier than iron. • These elements, and lighter ones such as carbon and oxygen, are ejected into space by the explosion. • As a supernova spews material into space, its core continues to collapse.

43. Life Cycle of Stars If the remaining core of a supernova has a mass less than about three times the sun’s mass, it will become a neutron star, the dense remnant of a high-mass star that has exploded as a supernova. • In a neutron star, electrons and protons are crushed together by the star’s enormous gravity to form neutrons. • Neutron stars are much smaller and denser than white dwarfs.

44. Life Cycle of Stars A neutron star spins more and more rapidly as it contracts. Some neutron stars spin hundreds of turns per second! • Neutron stars emit steady beams of radiation in narrow cones. • A spinning neutron star that appears to give off strong pulses of radio waves is called a pulsar.

45. Life Cycle of Stars Pulsars emit steady beams of radiation that appear to pulse when the spinning beam sweeps across Earth.

46. Life Cycle of Stars If a star’s core after a supernova explosion is more than about three times the sun’s mass, its gravitational pull is very strong. The core collapses beyond the neutron-star stage to become a black hole. A black hole is an object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot escape from it.

47. Groups of Stars A group of stars that appear to form a pattern as seen from Earth is called a constellation. The stars in a constellation are generally not close to one another. They just happen to lie in the same general direction of the sky as seen from Earth.

48. Groups of Stars Astronomers have determined that more than half of all stars are members of star systems. Most stars occur in groups of two or more. • A star system is a group of two or more stars that are held together by gravity. • A star system with two stars is called a binary star. The two stars orbit each other.

49. Groups of Stars Sometimes the smaller star in a binary star is too dim to be seen easily from Earth but can still be detected from the motion of the other star. If one star passes in front of the other, blocking some of the light from reaching Earth, the star system is called an eclipsing binary. The brightness of an eclipsing binary varies over time in a regular pattern.

50. Groups of Stars There are three basic kinds of star clusters: open clusters, associations, and globular clusters. Studying star clusters is useful because all the stars formed together in the same nebula, so they are about the same age and the same distance from Earth. Astronomers plot the stars of a cluster on an H-R diagram to estimate the cluster’s age.