In the Universe, there are more stars than anything else and since the beginning of time, their births have represented a natural process that is repeated over and over. When you think of a galaxy, think of stars. Galaxies are the home of stars—they are collections of billions of stars all held together by their collective gravity. There are no stars between galaxies.
Stars shine from light created as a by-product of nuclear fusion that occurs at their centers when hydrogen atoms are converted into helium atoms. For a star the size of our Sun, this process takes place in an area equal to 1.6% of the total volume of the star, but containing half of its total mass. The density in this region is so great that a photon (a packet of light) ricochets for about 170,000 years before reaching the surface.
Star shine brightly and give off tremendous amounts of energy because they convert massive quantities of hydrogen into helium. In our Sun, 700 million tons of hydrogen are converted to helium every second, but even at this rate our Sun can produce light for billions of years.
Normal size, mass, brightness, temperature and life
Stars come in all sizes, but even the smallest are large and massive. Our Sun is an average-size star. It has a diameter of 865,000 miles and a mass 330,000 times that of Earth. The diameter of normal-size stars varies from about 1/3 to 10 times the diameter of our Sun. Star mass, or the amount of matter that a star contains, varies from 1/10 to 40 times that of our Sun. Their brightness varies from 1/100 to over 1,000,000 times that of our Sun and the smallest stars have surface temperatures of just 5,000°F while the largest reach 70,000°F. Our Sun will last about 10 billion years, while the smallest stars may last a trillion and the largest only a million.
These ranges apply to normal stars. There are stars that are much smaller or much larger than mentioned above, but they represent special cases of stars that are at the ends of their lives. This includes stars known as white dwarfs, neutron stars, black holes, giants and supergiants.
Cepheid variables are a famous category of stars. Historically, these stars gave astronomers the “yardstick” tool for measuring the distances to many objects within our galaxy and even nearby galaxies (but not to the distant ones).
Variable stars change in brightness, which can take from a few days to a year or more. Cepheid variables are named after the first-known of it kind, observed in the constellation Cepheus, the King, visible in the northern sky. These types of variables can be either giant or supergiant stars near the ends of their lives. They slowly change in brightness over a period of a few days to months by expanding and contracting in size — their diameters changing anywhere from 5 to 10 percent. Polaris, the North Star, is a Cepheid variable, but its brightness changes very little. In 1912, Henrietta Leavitt discovered that the true brightness (compared to our Sun) of a Cepheid variable solely depends on its pulsation period. If the true brightness of a star is known, then it is easy (for astronomers) to calculate its distance. For a down-to-Earth example, if we took a 60 watt light bulb (a known brightness) and moved it to any distance, that distance could be found simply by measuring its new apparent brightness (the brightness of light falls off by a factor of exactly four when its distance is doubled). So, astronomers have used this discovery to measure the distances of Cepheid variables in star clusters, nebulae and even nearby galaxies.
When the Universe was formed, it created just two elements — hydrogen and helium. The other elements of the periodic table were produced by stars. These include metals and the various silicates or materials that make up rocks. Our Sun is a “second generation” star, and like the planets it contains elements created from stars that have lived and died. Some of the heavy elements, like gold, could only have been formed by supernova explosions.
Giants and supergiants
Really large stars are “puffed up” stars near the ends of their lives. Towards the end of our Sun’s life, it will expand to about the size of Mercury’s orbit and will be considered a red giant. Stars that are much more massive than our Sun become supergiants. Their outer atmospheres can expand to at least 1,500 times the diameter of our Sun (the diameter of Earth’s orbit is equal to 200 Suns). The outer atmosphere of the supergiant Betelgeuse in the constellation Orion would almost reach the orbit of Jupiter.
Stars live long lives, but eventually they will all “die” by running out of fuel for nuclear fusion.
The smallest live the longest
The smallest “normal” stars, which have masses about 1/10 that of our Sun, live long lives. These stars are called red dwarfs because they are small in diameter and red in color. They slowly and efficiently fuse hydrogen into helium for possibly a trillion or more years. Red dwarfs die quietly, becoming cold dark cinders. Our Universe is not old enough for a red dwarf to have died yet.
Stars the size of our Sun
Medium-sized stars like our Sun die in a different way. These stars burn hotter and faster than the smallest stars, so their lifespans are shortened to around 10 billion years. They end up with a core of helium and an outer shell of hydrogen that never got used for fusion. There is an incredible balancing act in the interiors of all stars. Stars are relatively stable because they balance the outward forces produced at their cores by the weight or mass of all the gas above them. However, with medium-sized stars, there comes a point when the helium core must contract to maintain balance. But in doing so, it heats up tremendously (the energy of the collapse produces the extra heat), so much so that it ignites the outer shell of hydrogen, causing it to fuse into helium and expand enormously — to diameters as large as Earth’s orbit. These medium-size stars become giant stars.
From planetary nebulae to white dwarfs
The thin outer atmospheres of giant stars eventually expand to form what are called planetary nebulae. The remains of the stars shrink to what are known as white dwarfs, very dense stars, about the size of Earth, with very energetic hot surfaces exceeding 45,000°F. This is hot enough to produce highly energetic ultraviolet light that not only pushes the gas around it outward, but also excites the gas to fluoresce, that is, to give off its own light.
Planetary nebulae have nothing to do with planets! It is an old name that stuck, used before anyone understood the true nature of these celestial objects. The word “planet” was originally coined because many of these nebulae are roundish, resembling planets to a degree. See pictures on the opposite page.
Planetary nebulae expand to 3 or more light years in size and eventually dissipate, perhaps lasting for 10,000 years. They are not always round because their shape can be affected by magnetic fields generated by the dying stars.
Dying Stars: Supernova Explosions
The last supernova in our galaxy occurred in 1604. They are rare events. The light they produce is anywhere from 600 million to 4 billion times the brightness of our Sun. They can remain bright for several months, but then fade over the course of a year. Since they actually shine brighter than an entire galaxy, astronomers and amateur astronomers look for them by scanning galaxies for very visible bright spots.
There are two types of supernova explosions. The fainter is produced when a massive star uses up its nuclear fuel. When this happens, the huge core collapses suddenly, sending out a shock wave that blasts the remaining outer atmosphere outward.
The brightest supernovae occur in a binary star system where two stars orbit extremely close to one another. This close proximity enables one of the stars, a white dwarf, to gravitationally pull in incredible amounts of atmospheric material from its companion. This eventually increases the mass of the white dwarf tremendously, forcing it to collapse, causing the element carbon (leftover material created by fusion in the original star before it “died” and became a white dwarf) to ignite in a furious nuclear fusion that consumes the whole star and produces the most violent explosion known in the Universe.
Neutron stars, black holes and pulsars
The fainter supernovae can create neutron stars or black holes depending on how much material or mass was left after their explosion. A neutron star is an object about 10 miles in diameter, with a sugar cube size piece of its material weighing in at 100 million tons. Pulsars are rapidly rotating neutron stars (rotating as fast as 1,000 times per second).
At one time, astronomers thought that a nova might be a moderated supernova. Well, after collecting and analyzing data on novae, they discovered that novae are quite different. A nova is like a repeating camera “flash.” Nova flashes occur in binary systems, where two stars are orbiting near one another. One of the stars, a white dwarf, gravitationally pulls off some of the outer atmosphere of its companion. When a substantial amount of the hydrogen atmosphere has accumulated on the white dwarf’s surface, it ignites like a nuclear fusion bomb. Both stars remain intact after the explosion, to begin the process anew. A nova flash can cause a 100,000 fold brightening. Like many situations in nature, a nova and the brightest supernova have similar setups (binary systems with white dwarfs), but small differences produce dramatically different outcomes.
The most mysterious objects in the Universe may conjure up thoughts of devouring monsters, but black holes are nothing more than fairly tame dead stars.
What exactly is a black hole?
The idea of a black hole was proposed shortly after Isaac Newton developed his famous equation about gravity in 1687. A black hole is a celestial object so dense, the gravity it produces will not even allow light to escape from its surface.
Small to large
Black holes come in a variety of masses, from low or “small” mass to supermassive. A low mass black hole is one that has a mass of at least three times that of our Sun. Supermassive black holes are believed to reside at the center of most, if not all, galaxies. These are estimated to have masses millions of times greater than that of our Sun.
Becoming a black hole — no application necessary
Low mass black holes can form from the remains of supernova explosions. If the mass of a star that remains after one of these explosions is three or more times that of our Sun, the object will automatically collapse to become a black hole. Anything less massive than three solar masses will become a neutron star or white dwarf. Our Sun does not have enough mass to become a black hole — it will become a white dwarf. A low mass black hole can only be created from stars much more massive than our Sun.
Supermassive black holes, located at the center of galaxies, may become very massive by pulling in extra matter, such as nebulae clouds, when galaxies collide.
Zero in size?
According to the mathematics, all black holes, no matter how massive, have zero diameters. This seems difficult to believe, so scientists are studying this idea further to see if black holes might have small diameters. But scientists are fairly sure of one thing, that the “smallest” black holes, those with just three times the mass of our Sun, would have a diameter smaller than the periods printed on these pages.
Although black holes may only be the size of periods at the end of sentences, they create around themselves a larger area of no return called an “event horizon.” If you enter the event horizon, you cannot escape from the black hole. The diameter of the event horizon increases with mass. A black hole with a mass of three of our Suns has an event horizon 11 miles in diameter, whereas a black hole with a mass of a million Suns has a diameter of 5,600,000 miles, which is 14 times smaller than Mercury’s orbit around the Sun.
Black holes are not vacuum cleaners
If our Sun suddenly became a black hole, the planets would continue to orbit the black hole just like they did the Sun. A black hole does not act like a vacuum cleaner and suck things into it! A spacecraft could visit a black hole safely by orbiting around it. If something gets close to a black hole, it will get pulled in, but this is no different than a spacecraft crashing into a planet if it comes too close without entering an orbit.
SUN shines very brightly at magnitude, –26.8
1. SIRIUS, (alpha), Canis Major, –1.4
2. CANOPUS, (alpha), Carina, –0.6
3. ARCTURUS, (alpha), Bootes, –0.1
4. RIGIL KENT, (alpha), Centaurus, –0.0
5. VEGA, (alpha), Lyra, 0.0
6. CAPELLA, (alpha), Auriga, 0.1
7. RIGEL, (beta), Orion, 0.2
8. PROCYON, (alpha), Canis Minor, 0.4
9. ACHERNAR, (alpha), Eridanus, 0.5
10. BETELGEUSE, (alpha), Orion, 0.5
11. HADAR, (beta), Centaurus, 0.6
12. ALTAIR, (alpha), Aquila, 0.8
13. ACRUX, (alpha), Crux, 0.8
14. ALDEBARAN, (alpha), Taurus, 0.9
15. SPICA, (alpha), Virgo 1.0
16. ANTARES, (alpha), Scorpius, 1.1
17. POLLUX, (beta), Gemini, 1.2
18. FOMALHAUT, (alpha), Piscis Austrinus, 1.2
19. BECRUX, (beta), Crux, 1.3
20. DENEB, (alpha), Cygnus, 1.3
21. REGULUS, (alpha), Leo, 1.4
22. ADHARA, (epsilon), Canis Major, 1.5
23. CASTOR, (alpha), Gemini, 1.6
24. GACRUX, (gamma), Crux, 1.6
25. SHAULA, (lambda), Scorpius, 1.6
26. BELLATRIX, (gamma), Orion, 1.6
27. ALNATH, (beta), Taurus, 1.7
28. MIAPLACIDUS, (beta), Carina, 1.7
29. ALNILAM, (epsilon), Orion, 1.7
30. ALNAIR, (alpha), Grus, 1.7
31. ALNITAK, (zeta), Orion 1.7
32. REGOR, (gamma), Vela, 1.7
33. ALIOTH, (epsilon), Ursa Major, 1.8
34. MIRPHAK, (alpha), Perseus, 1.8
35. KAUS AUSTRALIS, (epsilon), Sagittarius, 1.8
36. DUBHE, (alpha), Ursa Major, 1.8
37. WEZEN, (delta), Canis Major, 1.8
38. ALKAID, (eta), Ursa Major, 1.9
39. AVIOR, (epsilon), Carina, 1.9
40. SARGAS, (theta), Scorpius, 1.9
41. MENKALINAN, (beta), Auriga, 1.9
42. ATRIA, (alpha), Triangulum Australe, 1.9
43. ALHENA, (gamma), Gemini, 1.9
44. DELTA VELA, (delta), Vela, 1.9
45. PEACOCK, (alpha), Pavo, 1.9
46. POLARIS, (alpha), Ursa Minor, 2.0
47. MIRZAM, (beta), Canis Major, 2.0
48. ALPHARD, (alpha), Hydra, 2.0
49. NUNKI, (sigma), Sagittarius, 2.1
50. ALGOL, (beta), Perseus, 2.1
51. DENEBOLA, (beta), Leo, 2.1
52. HAMAL, (alpha), Aries 2.1
53. ALPHERATZ, (alpha), Andromeda, 2.1
54. KOCHAB, (beta), Ursa Minor, 2.1
55. SAIPH, (kappa), Orion, 2.1
56. DENEB KAITOS, (beta), Cetus, 2.1
57. ALSUHAIL, (lambda), Vela, 2.2
58. ASPIDISKE, (iota), Carina, 2.2
59. ALPHEKKA, (alpha), Corona Borealis, 2.2
Note: Venus and Jupiter always outshine the brightest nightime stars. Venus hovers around magnitude –4 and Jupiter around –2.