Astronomy — Stars
The Stars
- Star distances: Sirius is 9 light-years, Denebola is 40 light-years, Alphecca in Corona Borealis is 75 light-years, Regulus is 90 light-years, Altair is 16.3 light-years, Spica is 250 light-years, Shaula (the stinger of Scorpius) is 333 light-years, Deneb is 1600 light-years, M13 in Hercules is 25,000 light-years, and Betelgeuse is 520 light-years.
- Some stars are thousands of times larger than our Sun.
- The ‘twinkling’ of a star is caused by distortion of the star’s light as it passes through our atmosphere. A star near the horizon, where we look through more air, will twinkle more than a star overhead.
- More than half of all stars are multiple-stars:
- Double stars: they appear close only because of the line of sight from Earth.
- Binary stars: the stars actually orbit each other.
- Some binaries take 100’s or 1000’s of years to compete one revolution.
- Visual binaries: the two stars are separately visible in a telescope.
- Sirius is a visual binary. It has a white dwarf companion. The orbital period is 50 years.
- Spectroscopic binary: stars that are so close together they can only be differentiated spectroscopically.
- Capella is a spectroscopic binary, as is Alcor (21 parsecs away).
- Mizar is 22 parsecs away and is a double-double, that is, each of the visible stars is a spectroscopic binary.
- 61 Cygni is a true visual binary, with a period of 653 years, at a distance of 3.4 parsecs. One of the stars of 61 Cygni has a planet-like companion eight times the mass of Jupiter.
- Eclipsing binaries: binaries whose stars cross in front of each other as seen from earth. They are rare but they allow measurement of a star’s diameter. Algol is the most famous example. It eclipses every 69 hours.
- Albireo (the Cub Scout star, β Cygnus) is not a binary system. It has a yellow K3 and a blue B8.
- Temperature and color: the colors range from blue (the hottest) down to red (the coolest). The various colors, from hottest down to coolest, are blue, white, yellow, orange, and red. Blue stars include Rigel, Spica, and Sirius; yellow stars include our Sun and Capella; orange stars include Aldebaran and Arcturus; red stars include Betelgeuse and Antares.
- Visual magnitude of a star depends on temperature, size, and distance from Earth.
- Luminosity classes:
- Ia – bright supergiant (Rigel, β Orion)
- Ib – Supergiant (Polaris)
- II – bright giant (Adjara, ε Canis Major)
- III – giant (Capella, α Auriga)
- IV – subgiant (Altair, α Aquila)
- V – main sequence star (Sun)
- Main sequence stars, like our Sun, have dense atmospheres and their spectral lines are smeared and broad. Giant and supergiant stars have thin atmospheres and their spectral lines are sharp and narrow.
- Betelgeuse is only half as hot as our Sun but 500 time bigger in diameter. It is 520 light-years away.
- The genealogy of a star:
- Thin clouds of hydrogen begin to feel the force of gravity
- A pocket of gas forms, about 1.6 light-years in diameter
- Gravity, along with heat and energy, cause this pocket to contract
- A ball gas, or embryonic star, forms
- Gravity continues its influence, and temperatures rise to 100,000 degrees Fahrenheit
- Electrons and protons form, with the diameter of the sphere about 100 million miles
- Nuclear fusion begins, with temperatures increasing to 20 million degrees Fahrenheit
- Helium atoms are formed, with diameter of the sphere now 1 million miles
- Heat and light begin to be emitted from the sphere
- Reddening and expansion occur, forming a red giant
- Hydrogen becomes exhausted, temperature is now 200 million degrees Fahrenheit
- Collapse of the sphere begins
- Helium fuses to carbon, and the collapse halts
- Helium is exhausted, and collapse begins again
- At this point, smaller stars die and become white, compact dwarfs, with 1 teaspoon of material weighing 10 tons. Heat continues to dissipate athe eventually a black corpse is formed with a radius of 10,000 miles.
- Larger stars explode at 600 million degrees Fahrenheit, carbon fusion occurs, fusion of other elements us to iron occurs, a supernova explosion occurs, and either a neutron star or a black hole forms.
- The first 8 steps can take over 10 million years; the next 6 steps can take 100 million years.
- Neutron stars: A supernova explosion leaves behind a neutron star. They are as dense as atomic nuclei and have interior temperatures of 1 billion degrees. Incredibly strong electric and magnetic fields surround them They revolve up to 1000 times per second. A square centimeter of their surface radiates a billion billion times more energy than a square centimeter of the Sun. They are so dense that 1 thimbleful of its matter would weigh more than all the cars on Earth. A neutron star is the core of a star that has collapsed to a radius of about 6 miles and a density so high that only neutrons exist. The original star mass is between 1.4 and 3 solar-masses. A sugar-cube-sized lump of this material would weigh 100 million tons on Earth. They are very hot, spin rapidly (30-100 times per second), and have strong magnetic fields. They are also called pulsars, and are commonly found inside supernova remnants.
- Variable stars are denoted by a capital letter before the constellation name, beginning with R and going through Z. Double capital letters are used also.
- Light is blue-shifted (shorter wavelength) as it approaches, and red-shifted (longer wavelength) as the source moves away.
- The spectral classes move from O (large mass, high luminosity, high temperature) down to M (lower mass, lower luminosity, lower temperature). The classes are O, B, A, F, G, K, M (Oh, be a fine girl, kiss me). M stars are very common, O stars are very rare (only 1 star in 4 million is an O star).
- Astronomers divide each spectral class into 10 subclasses. Our Sun is a G2 star.
- The most massive stars are 55 to 100 times the mass of our Sun, and the least massive ones are one-tenth of our Sun’s mass.
- A star’s location along the main sequence depends on its mass. The more massive a star is, the more luminous it is.
- Luminosity of a star (compared to our Sun) = (solar masses of the star)3.5
- Main sequence stars have average densities, similar to our Sun (1 gram / cm3), but giant stars have low densities ( 0.1 – 0.001 grams / cm3), and super-giants are 0.001 – 0.00001 grams / cm3). These densities are thinner than the air we breath, and if we could insulate ourselves against the heat, we could fly an airplane through these giants.
- White dwarfs have masses similar to our Sun, but are only as large as the Earth. Their density is equal to 108 grams / cm3. On Earth, 1 cubic centimeter of this material would weigh 20 tons. Sirius B is a white dwarf. They are smaller than Earth, but weigh as much as the Sun. Theya re probably crystalline carbon (diamond).
- The Great Nebula in Orion is 8 parsecs across and 4 brilliant white stars know as the Trapezium lie in the center. The largest of the four is 40 solar-masses and 300,000 times more luminous than our Sun. The Great Nebula is nearly a vacuum, containing only 600 atoms per cubic centimeter. For comparison, the interstellar medium has an average density of 1 atom per cubic centimeter, and air at sea level has 1010 atoms per cubic centimeter.
- Black holes: Black holes are not giant vacuums. A black hole is a gravitational field, and at a reasonably large distance its force is quite small. If the Sun were replaced by a 1 solar-mass black hole, the orbits of the planets wouldn’t change at all. Black holes are strong sources of X-rays. That source may be a black hole into which matter is falling. Any compact body in an X-ray binary with a mass greater than 3 solar masses must be a black hole.
- Novae occur in close binary star systems in which one star is usually a white dwarf and the other is a star evolving off the main sequence. As the evolving star expands, some of its hydrogen-rich envelop is drawn to the white dwarf. As this material accretes on the white dwarf’s surface, its temperature increases until thermonuclear reactions begin. The reactions proceed explosively, creating the sudden brightening we see here on Earth. At its height, a typical nova shines about 100,000 times brighter than the quiescent binary system. The system then returns to normal and the process starts again.
- In stars, pressure and gravity act against each other. If gravity wins, the star contracts. If pressure wins, the star expands.
- Spica is a B1 main-sequence star.
- Stars form from interstellar medium, the gas and dust floating between stars. This medium is 75% hydrogen and 25% helium with other trace elements and dust grains being present. The average distance between dust grains is 150 meters. Cool clouds have between 10 and 1000 atoms per cubic centimeter. Hot clouds have 0.1 atoms per cubic centimeter. As starlight passes through these gas clouds, red photons (longer wavelength) are less likely to be reflected than blue photons (shorter wavelength), so we see interstellar reddening. The most dense interstellar dust clouds contain 1000 atoms per cubic centimeter, including several hundreds or thousands of solar masses, and have a temperature of 10 degrees Kelvin. These clouds don’t collapse and form stars until they collide with a shock wave formed by a supernova explosion or the spiral arm rotation of a galaxy.
- The more massive a proto-star is, the faster it contracts. It took our Sun 30 million years to contract. A large proto-star can contract in 160,000 years. Stars probably form planets as they contract. When a contracting star becomes hot enough to start fusing hydrogen, it stops contracting.
- The matter in a star is supported against its own gravity by the nuclear reactions at the core, fusion reactions that combine hydrogen nuclei (protons) into helium nuclei (2 protons and 2 neutrons).
- Proton-proton fusion -> 10 million degrees Kelvin; C-N-O cycle fusion -> 16 million degrees Kelvin and requires 1.1 solar masses; helium fusion -> 100 million degrees Kelvin; carbon fusion -> 600 million degrees Kelvin.
- The temperature at the center of a 15 solar-mass star is 34 million degrees Kelvin, twice the central temperature of our Sun.
- The four laws of stellar structure
- Continuity of mass – mass bust be distributed smoothly throughout a star
- Continuity of energy – energy leaving a star equals energy produced in the star
- Hydrostatic equilibrium – the weight of the mass pressing down is balanced by the pressure pushing out. Also, pressure and temperature must increase from surface to core
- Energy transport – energy flows from hot regions to cooler regions by conduction, convection, or radiation
- The average star spend 95% of it life fusing hydrogen on the main sequence. High mass stars have short lives (a few million years) and die in tremendous explosions, leaving neutron stars or back holes. Low mass stars have long lives (hundreds of billions of years) and die quiet deaths, leaving white dwarfs.
- T Taurus stars are stars in the later stages of contraction, approaching main sequence.
- Aldebaran has a diameter 25 times greater than our Sun’s diameter but it is only half as hot.
- Death of a giant star (3 to 9 solar masses):
- Expansion into a giant – hydrogen fuses to form helium at greater than 100,000 degrees Kelvin, helium ash builds up, the core contracts as hydrogen is used up and temperature increases, the hydrogen fusion shell creeps outward, and the star expands.
- Helium fusion – when the temperature reaches 100,000,000 degrees Kelvin, helium fuses to form carbon and oxygen.
- Carbon fusion – at 600,000 degrees Kelvin
- Heavy element fusion – ends when the star forms an iron core because the iron nucleus is the most tightly bound of all nuclei
- Collapse of the core – the collapse triggers a star-destroying supernova explosion.
- The Crab Nebula in Taurus marks the site of the 1054 A.D. supernova. It is 2.5 parsecs in diameter.
- A nova is the eruption of a white dwarf.
- Death of a low mass star: they die relatively quietly as they exhaust their nuclear fuel. The most violent event in the lives of these stars is the ejection of their surface layers to form planetary nebulae.
- Ursa Major: five of the seven bright stars in the Big Dipper belong to a single star cluster, the Ursa Major moving group. The cluster lies about 75 light-years away, making it the nearest known star cluster to Earth.
- Polaris: if you measure its altitude above the horizon, you know your latitude. A supergiant star, Polaris is a Cepheid variable. Such stars change brightness regularly due to a swelling and shrinking of the star itself. Polaris changes only slightly with a period of 4 days.
- Mu (μ) Cephei is a red supergiant, famous for its color. It is the reddest star in the northern sky, hence its name, the Garnet Star.
- Astrologers once considered Algol (β Perseus) to be the most dangerous, violent, and unlucky star in the heavens. It has been called the Ghoul star, because it represents Medusa’s head. Algol is an eclipsing binary, with two stars orbiting in our line of sight. Algol normally is a s bright as Mirfak (α Perseus) but every 2 days 20 hours 49 minutes, it dims noticeably and stays dim for ten hours. The fainter of the two stars blocks light from the brighter one.
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