ASTRO 7N Exam 2025

Sun's composition - -Composed mostly of gas - hydrogen (73% by mass) and helium (25% by mass)` Sun's stability - -Stable due to balance of gravity (inward force) and gas pressure in its hot interior (outward force) Sun's gas pressure - -Gas pressure provided by nuclear fusion reactions in the Sun's core, going on at a temperature of 15 million degrees Kelvin Sun's diameter and mass compared to Earth - -Diameter: 109*Earth Mass: 333,000 * Earth Sun's luminosity - -Equivalent to 4 * 10^24 100 Watt light bulbs Solar system's mass and the sun - -99.99% of all the mass of the solar system is in the Sun alone How long has the sun been shining for? - -Sun has been shining for 4.5 billion years, and will continue for about another 5.5 billion years Sun and sunspots - -Sunspots are slightly-cooler regions on the Sun's surface due to magnetic activity preventing hot material from rising in that region; have 11 year cycles Nuclear fusion - -A long-lasting source of energy in stars; 4 protons combine to make helium-4 (2 protons and 2 neutrons) and release energy in gamma rays ` Proton-Proton chain step 1 - -Two protons collide at very high speed, and stick together; one of them changes into a neutron; ends with a Deuterium nucleus (2H : 1 proton and 1 neutron, bound together), plus released energy Proton-Proton chain step 2 - -Deuterium nucleus from step 1 collides with another proton, and makes a Helium-3 nucleus (3 He : 2 protons and 1 neutron, bound together), plus some more excess energy released Proton-Proton chain step 3 - -two Helium-3 nuclei combine to make a Helium-4 nucleus (4 He : 2 protons and 2 neutrons, bound together), releasing back 2 protons in the process and some more extra energy Although it requires a lot of energy initially to cause the high-speed collisions between protons and nuclei during each step of the "p-p chain,": - -A little bit of extra energy is generated and released in every step ASTRO 7N ASTRO 7N The energy output from the p-p chain comes from: - -E = mc^2 The mass of a Helium-4 nucleus is less than: - -the mass of 4 protons; some of that mass is converted into energy (in the form of gamma - ray photons) Layers of the sun: Core - -At the center; high density and temperature; where nuclear reactions occur and gamma rays are produced Layers of the sun: Radiative zone - -Photons are repeatedly re-absorbed and re emitted; the energy of an individual photon can take on average 170,000 years to pass through Layers of the sun: convective zone - -hot gas rises and cold gas sinks; light traverses in about 1 week Layers of the sun: Photosphere - -temperature 5780K; this is the "surface" of the Sun that we see; photons have been converted to visible wavelengths; can see "granules" due to convection bringing material up and down in cells Layers of the Sun: Chromosphere - -Red or orange in color; temperature about 4,500 K; we see through this, down to the photosphere Layers of the Sun: Flare - -An eruption coming out of the Sun due to magnetic activity Layers of the Sun: prominence - -A hoop-shaped eruption out of Sun due to magnetic activity Layers of the Sun: Corona - -Low density; temperature about 1 million K; visible during solar eclipse Layers of the Sun: Solar Wind - -Charged particles coming from the Sun' surface, escaping to deep space; the solar wind permeates the whole Solar System Luminosity (L) - -The absolute power output, at the source (e.g., a star's surface) Brightness (B) - -The apparent output, as observed some distance (d) away Inverse-Square law - -Determines how bright the star appears, based upon its luminosity (intrinsic brightness ) and its distance; B = L / (4*pi*d^2) or B is inversely proportional to L / d^2 If two stars have the same luminosity and one is ten times farther away than the other, it will appear: - -1/10^2 times as bright (that is, 100 times fainter). Another example: If star ASTRO 7N ASTRO 7N A is 4 times as luminous as star B, then stars A and B would appear equally bright if star A were 2 times as far away as star B Parallax Method to Measure Distances: Basic Concept - --The basic concept is to view a star from two locations on opposite sides of Sun (Earth, but 6 months apart), and look for minute changes in its apparent position - These locations in Earth's orbit are on opposite ends of the "baseline" - This is like holding something - say, your finger - up in front of your face, and closing or covering one eye and then the other; your finger will appear to move back and forth a lot more than objects in the background in your field of vision. More on the Parallax Method to Measure Distances - -- Some simple small-angle trigonometry can approximate distances to nearby stars - For stellar parallax measurements, the baseline is 1 AU (average distance between the Earth and Sun) Parallax Method to Measure Distance: target star - -The target star appears to move (relative to more-distant stars) by an angle 2 times the parallax angle; so D = 1/p D is the distance in parsecs, p is the parallax angle in arcseconds (1/3600 degree) Parallax method examples - -- A star with measured parallax angle of 0.1 arcsec is 1 / 0.1 = 10 parsecs away - A star with a parallax angle of 0.02 arcsec is 1/0.02 = 50 parsecs away Sometimes a star that appears fainter is still closer than one that appears brighter, because - -the star that appears brighter is actually more luminous Hertsprung-Russell Diagram: horizontal axis - -Star's surface temperature, increasing from right to left - Cooler stars are redder (temperatures down to around 2,300 K or so) - Hotter stars are bluer (temperatures up to around 40,000 K or so) Hertsprung-Russell Diagram: Spectral class - -Stars' effective surface temperature is estimated using its spectral class - the major spectral classes of stars, from hotter to cooler, run O-B-A-F-G-K-M - A popular mnemonic is "Oh be a fine girl/guy, kiss me" or "Only bored astronomers find gratification knowing mnemonics: - Spectral class is determined using the absorption spectrum, where absorption lines from different chemicals elements with different levels of ionization arise at different temperatures ASTRO 7N ASTRO 7N Hertsprung-Russell Diagram: vertical axis - -luminosity, expressed in terms of the luminosity of the Sun ( "L⊙" ) Nearest stars: Alpha Centauri - -Alpha Centauri is the closest star, 4.3 light years (or 1.35 parsecs) from the Sun; actually part of a triple star system, with its brightest member similar to the Sun - Most of the nearest stars are cool and dim, and fall on the lower right of the H-R diagram; this is because most stars in general have these properties. Brightest Stars: Sirius (the "Dog Star") - -Twice as massive as the sun; it has a binary companion star, a white dwarf The "brightest stars" (that is, as we see them from Earth) - -Have more varied properties - red and blue, low and high luminosity, with some on the lower right of the H-R diagram, but others near where the Sun is, some on the upper left, and some on the supper right (red giant region) The "brightest stars" tend to be biased towards - -stars that already have high luminosities, so that they appear bright to us even at large distances; we simply cannot see low-luminosity stars if they are too far away (even if greater in number) Properties of a star - -- Luminosity, mass, size, temperature and age - Size and temperature directly affect luminosity, as in: L is proportional to R^2 * T^4 -- A larger size = larger light-emitting surface area = greater luminosity --Higher temperature = much greater luminosity (also peaks in bluer colors) Main sequence stars - -- Main sequence stars location on H-R diagram in a band from lower right across to upper left - around the slightly-wavy red line in the example H-R Diagram - When on the main sequence (which occupies the majority of a star's lifetime), star are burning Hydrogen into Helium in their cores (by the p-p chain) -For stars on the main sequence, higher temperature stars have a higher luminosity Where are red dwarfs and blue giants location on the H-R diagram? - -Red dwarfs (cool, faint, and small) on lower right; blue giants (hot, bright, and large on upper left) What determines where on the main sequence a star lives? - -Mass determines where on the main sequence a star lives, and what the main sequence lifetime is for the star; more-massive main sequence stars are on the uppoer left of H-R diagram; masses range from about 0.1 to 100 times the mass of the Sun; sizes range from 0.1 to 15 times the radius of the Sun Main sequence stars luminosities - -Luminosities of main sequence stars range from 10^-3 to 10^6 times that of the Sun ASTRO 7N ASTRO 7N Main star sequence stars age - -Ages range from a few million years for more massive stars, to much more than 14 billion years - the current age of the Universe - for less massive stars. The more massive stars use their greater fuel supply more rapidly stars not on the main sequence: red giants and supergiants - -- Red giants and super giants: burning helium or even heavier elements in their cores (not hydrogen anymore); starting to die; size is large, so they are very luminous even though they are relatively cool; top right of H-R diagram Stars Not on the Main Sequence: White dwarfs - -- Wh