Our modern definition of a star is a large ball of gas that generates energy by nuclear fusion in its hot central co e. Our Sun shines as a star because it fuses
hydrogen into helium in its core.
proto star before the star’s center gets hot enough to ignite nuclear fusion, the unfinished star
life cyclebegins with the star’s formation in a giant cloud of gas. A star is “born” when nuclear fusion begins in its central core. A star “dies” when it finally ceases
to produce energy by any kind of fusion.
Stars shine fairly steadily during this period, brightening gradually as they age
a star that exhausts its core hydrogen begins to grow larger and brighter, becoming a giant supergiant star. For example, when our own Sun becomes a red
giant some 4–5 billion years from now, it will grow so large that it will engulf some of the inner planets.
Stars can begin to fuse heavier elements during their giant or super giant stages.
Eventually, the star will reach a point where it can fuse no other ele ments, and at that point the star dies.
Lowmass starsdie relatively gently, leaving white dwarf
Highermass starsdie in titanic explosions calledsupernovae; cores collapse to form neutron stars black holes
Searching for habitable planets: longlasting, hydrogenfusing stage of stars’ lives
Luminosity (the total amount of light a star emits into space) also depends on distance (apart from brightness) Spectroscopy: an invention to learn more about stars’ properties
the women astronomers of Harvard College Observatory made many great discoveries
at the beginning, Pickering’s sequence of spectral types A to O: using type A to designate stars with the strongest hydrogen lines, type B for those with slightly
fainter lines, and so on down the alphabet to type O for stars showing the weakest lines.
But Redundancies and the spectra fell into a much more natural order than he had supposed
Then, the spectral sequence: 7 major spectral types, OBAFGKM (Oh, Be A Fine Girl, Kiss Me); subdivided by number
Our Sun: G2
The system of stellar classification adopted in 1910
1925, PayneGaposchkin showed that the differences in the spectral types reflected differences in the surface temperatures of the stars.
Hertzsprung–Russell dia grams : graphs of the relationship between luminosity and surface temperature
All stars are born with basically the same composition, which is almost entirely (98% or more) hydrogen and helium
During the hydrogenfusing phase of its life, a star’s surface temperature (and hence its spectral type) and total luminosity are determined primarily by one thing:
the star’s mass
There is an enormous range in luminosity, and a much smaller range in mass
The rule is a general one:The more massive the star, the shorter its lifetime
Which stars make good sun? O, B (No, short lifetimes for the formation of planets and for life to take hold)
A,F (quite possible deep underground, subsurface oceans or ice. Or if they have a thick atmosphere or atmosphere containing sufficient oxygen) (fair lifetimes;
hotter than the sun, so habitable zones would be wider and would lie at greater distances from their central star; BUT higher temperatures mean much ultraviolet
light, which easily breaks chemical bonds in complex organic molecules) (complex plants and animals on Earth did not arise until our planet was some 4 billion
years old, the 1 to 2billionyear lifetime of A and F stars suggests that life on these planets would most likely be much simpler than life on Earth)
K,M (long life but the habitable zone becomes progressively smaller and closerin for stars of lower luminosity)
(two objectionsplanets orbiting close to a star get locked into synchronous rotation, with one side of the planet perpetually facing the star; small stars have
frequent and energetic flares*—sudden bursts of intense light and radiation—that might cook any complex life.)
neither of these objections is fatal (atmosphere circulates heat; protective layer of atmospheric ozone)
an object with a mass less than 8% that of the Sun, gravity isn’t strong enough to compress the core to high enough temperatures to sustain hydrogen fusion
substellar objects are called brown dwarfs
very dim and difficult to detect (easy with infrared telescopes as they emit moderate amounts of infrared radiation)
BUT have much hotter surfaces than planets
Have no habitable zones at all, but could have planets orbited by Europalike moons with subsurface oceans and perhaps life.
multiple star systems: in which two or more stars orbit each other closely
binary star systems with just two stars (more common)
there are really only two basic ways to search for extrasolar planets:
1.Directly: Pictures or spectra of the planets themselves constitute direct evidence of their existence.