Unit 1.2: Characteristics of Stars. Guiding Questions for this unit:. What two forces must be balanced for a star to achieve stability? What are the stages in the life cycle of a star? What determines whether a star will become a white dwarf, a neutron star, or a black hole?
Guiding Questions for this unit: What two forces must be balanced for a star to achieve stability? What are the stages in the life cycle of a star? What determines whether a star will become a white dwarf, a neutron star, or a black hole? What three properties of a star influence its brightness? How are parallax and Cepheid variables used in astronomy?
Vocabulary: Nebula: interstellar clouds of dust/gases. Protostar: “proto-” = early; a “star” that condenses due to gravity but does not yet undergo nuclear fusion Apparent magnitude: the brightness of a star when viewed from Earth. Absolute magnitude: the brightness of a star when viewed from 32.6 light years away. Parallax: the apparent displacement of an object when seen from two different perspectives; used to calculate distance to a star Cepheid variable: a star that varies in size and brightness due to expansion and contraction of the star core.
I. Evolution of a Star • Nebula: Stars begin in a nebula, an interstellar cloud of dust/gases. • Gases: mostly hydrogen and helium • Dust: mostly Carbon and Silicon (from supernova explosions of older stars) • Begin to condense through gravity • Possibly by shock-wave of nearby supernova? • As gravity increases, dust spins faster, gets hotter • Protostar: When a nebula has enough mass, it becomes a protostar. • “proto-” = early • Begins to radiate energy in red spectrum • But cannot yet undergo nuclear fusion
Main-sequence star: If a protostar has enough energy to begin nucleosynthesis, it becomes a main-sequence star: • Hydrogen fusion (nucleosynthesis) begins when star reaches high temperatures (hi temps = lots of energy) • Hi temps = more energy = increased motion = more outward pressure • But gravity pushes inward on star, causing collapse • Star becomes stable when balance between gas pressure and gravity is equal, main-sequence star
Red GiantStage: • Hydrogen-burning migrates away from inner core, progresses into outer shell • Increased pressure pushing outward causes expansion of star • Eventually gravity balances out this pressure expansion • Stabilizes but as a much larger star • Burning hydrogen cools as star expands resulting in a reddish appearance • Core continues nuclear fusion of helium into heavier elements • Up to element 26 (iron)
Death of a Star:- 3 Alternatives: • White dwarf: stars of low to medium mass • When fuel (H or He) is spent, no outward pressure, so gravity forces it to collapse into “degenerate matter” • Degenerate matter: electrons pushed close to nucleus, but repulsion prevents total collapse • Very dense: size of Earth, but density greater than the Sun
Neutron Stars: • Result from supernova events: large stars that explode (Chinese recorded supernova event in 1054 AD) • Release space dust that can be incorporated into new nebula/stars • Electrons are pushed so close to nucleus, they are forced to combine with protons, forming… neutrons • “Shell” of star is ejected, but core collapses into extremely dense, almost invisible object • Strong magnetic field ‘pulses’, allows us to “see” where a neutron star is located; called a ‘pulsar’
Black Holes • Largest (most massive) stars collapse into smallest objects b/c stronger gravitational attraction • Gravity is so strong in these objects that light cannot escape so cannot be seen • Pulls in material from surrounding stars, which can be viewed through x-ray lens (like that found on NuSTAR)
II. Properties of Stars • Brightness: The brightness of a star is influenced by temperature, distance, and size • Apparent magnitude: how bright a star appears from Earth. • Brighter stars have lower # (can even be negative), dimmer stars have higher # • ex: Sun has apparent magnitude of -26.7; Sirius: -1.4; Betelgeuse: 0.8 ... Bright, hot stars that are further can appear to be as bright as dim, cool stars that are closer to us.
Absolute magnitude: brightness of a star seen from 32.6 light years away (10 parsecs) • Allows scientists to “normalize” stars, compare them from an equal distance, so brightness depends on temperature and size of star, not distance • To compare absolute magnitudes, scientists must be able to calculate distance to stars
Use parallaxto calculate distance: • Determine position of Star A in comparison to Star B. Six months later, when Earth is at opposite end of its orbit around the Sun, determine how the position of Star A has changed in relation to Star B. • Using geometric functions, can calculate distance to Star A. • Closer starts have a greater parallax (larger angle shift) than more distant stars.
When stars are far, parallax shift is too small to detect. • Instead, use Cepheid variables: • Stars that pulsate/vary in size and brightness over a defined period of time, usually days • When gas pressures build up in the star, it expands, appears brighter. As star expands, less pressure pushing out, so gravity causes star to shrink, appear dimmer. Cycle repeats. • Brighter Cepheids have longer cycles; dimmer Cepheids have shorter cycles • All cepheids of same period have same brightness; can use that value to determine distance
Color: gives clues to a star’s temperature • Blue stars: very hot temperatures >30,000 Kelvin (K) • Most energy = shorter wavelengths = blue • Yellow stars: temps between 5000 – 6000 K (our Sun) • Red stars: coolest temperatures <3500 K • Least energy = longer wavelengths = red
Stellar Mass: the mass of a star provides information about its eventual demise • Mass is determined using binary star systems - two stars that orbit around each other. • If you know the size of their orbits, gravitational constants can help you calculate the masses of each star. • If both are same size, the center of mass lies halfway in between • If unequal, center of mass is closer to more massive star
Hertzsprung-Russell Diagram (H-R diagram) • Classifies stars according to absolute magnitude and surface temperature • Main-sequence stars: ~90% of stars, fall in a band that runs from upper left corner to lower right corner • Red Giants/Supergiants: in upper right corner • Same temp. as red stars, but much brighter, so must be much larger • (White) Dwarfs: below main-sequence stars • Same temp. as white stars, but much dimmer, so must be smaller