Reading Unit 55, 56, 58, 59

Sun’s magnetism is due to A. iron core of the Sun B. heating of Corona by energetic particles generated during Solar Flares C. generation of magnetic fields via fluid+magnetic field motions D. neutrino flows coming from the Sun ’ s core

Why was adaptive optics developed? a. To compensate for chromatic aberration b. To prevent distortion of mirrors by the vacuum of space c. To compensate for the image distortion caused by the Earth atmosphere d. To prevent fractures of the main mirror.

The PRIMARY reason for spreading many radio telescopes across a large area and combining the signals at a central station (i.e. combining radio telescopes to form an interferometer) is a. to produce a much sharper images of radio sources b. to avoid interference between signals from separate telescopes c. to be able to send a more powerful signal to space d. ensure that cloudy weather only affects a few of telescopes, leaving the others to continue observing

The main absorber in the atmosphere for infrared radiation, which impedes observations of astronomical infrared objects, is a. electrons in the Earth's atmosphere b. dust in the Earth atmosphere c. oxygen and nitrogen, the major constituents of the atmosphere d. water vapor

Pieces of metal are heated by varying amount in a flame. The hottest of these will be the one that shows which color most prominently? a. blue b. yellow c. red d. black

To a physicist a blackbody is defined as an object which a. absorbs all radiation which falls upon it b. always appears to be black, whatever its temperature c. always emits the same spectrum of light, whatever its temperature d. reflects all radiation which falls upon it, never heating up and always appearing black.

The specific colors of light emitted by an atom in a hot, thin gas are caused by a. protons jumping from level to level b. an electron dropping into the nucleus, producing small nuclear changes c. electrons jumping to lower energy levels, losing energy as they do so d. the vibrations of the nucleus

When electromagnetic radiation is Doppler-shifted by motion of the source away from the detector a. the measured wavelength is longer than the emitted wavelength b. the measured frequency of the radiation remains the same, but its wavelength is shortened, compared to the emitted radiation c. the speed of the radiation is less than the emitted speed d. the measured frequency is higher than the emitted frequency.

You see this every day! More distant streetlights appear dimmer than ones closer to us. It works the same with stars! If we know the total energy output of a star (luminosity), and we can count the number of photons we receive from that star (brightness), we can calculate its distance Some types of stars have a known luminosity, and we can use this standard candle to calculate the distance to the neighborhoods these stars live in.

Photons in Stellar Atmospheres Photons have a difficult time moving through a star ’ s atmosphere If the photon has the right energy, it will be absorbed by an atom and raise an electron to a higher energy level Creates absorption spectra, a unique “ fingerprint ” for the star ’ s composition. The strength of this spectra is determined by the star ’ s temperature.

Stellar Surface Temperatures Remember from Unit 23 that the peak wavelength emitted by stars shifts with the star ’ s surface temperatures –Hotter stars look blue –Cooler stars look red We can use the star ’ s color to estimate its surface temperature –If a star emits most strongly in a wavelength (in nm), then its surface temperature (T) is: This is Wien ’ s Law

Measuring Temperature using Wein’s Law

Spectral Classification Around 1901, Annie Jump Cannon developed the spectral classification system –Arranges star classifications by temperature Hotter stars are O type Cooler stars are M type New Types: L and T –Cooler than M From hottest to coldest, they are O- B-A-F-G-K-M –Mnemonics: “ Oh, Be A Fine Girl/Guy, Kiss Me –Or: Only Bad Astronomers Forget Generally Known Mnemonics

Interferometry Stars are simply too far away to easily measure their diameters! –Atmospheric blurring and telescope effects smear out the light Can combine the light from two or more telescopes to pick out more detail – this is called interferometry –Two telescopes separated by a distance of 300 meters have almost the same resolution as a single telescope 300 m across! Speckle interferometry uses multiple images form the same telescope to increase resolution

The Stefan-Boltzmann Law The Stefan-Boltzmann Law links a star’s temperature to the amount of light the star emits –Hotter stars emit more! –Larger stars emit more! A star’s luminosity is then related to both a star’s size and a star’s temperature

A convenient tool for organizing stars In the previous unit, we saw that stars have different temperatures, and that a star’s luminosity depends on its temperature and diameter The Hertzsprung-Russell diagram lets us look for trends in this relationship.

The H-R Diagram A star’s location on the HR diagram is given by its temperature (x-axis) and luminosity (y-axis) We see that many stars are located on a diagonal line running from cool, dim stars to hot bright stars –The Main Sequence Other stars are cooler and more luminous than main sequence stars –Must have large diameters –(Red and Blue) Giant stars Some stars are hotter, yet less luminous than main sequence stars –Must have small diameters –White Dwarf stars

The Family of Stars

Stars come in all sizes…

The Mass-Luminosity Relation If we look for trends in stellar masses, we notice something interesting –Low mass main sequence stars tend to be cooler and dimmer –High mass main sequence stars tend to be hotter and brighter The Mass-Luminosity Relation: Massive stars burn brighter!

Massive stars burn brighter L~M 3.5

Luminosity Classes

Stellar Evolution – Models and Observation Stars change very little over a human lifespan, so it is impossible to follow a single star from birth to death. We observe stars at various stages of evolution, and can piece together a description of the evolution of stars in general Computer models provide a “fast-forward” look at the evolution of stars. Stars begin as clouds of gas and dust, which collapse to form a stellar disk. This disk eventually becomes a star. The star eventually runs out of nuclear fuel and dies. The manner of its death depends on its mass.

Evolution of low-mass stars

Evolution of high-mass stars

Tracking changes with the HR Diagram As a star evolves, its temperature and luminosity change. We can follow a stars evolution on the HR diagram. Lower mass stars move on to the main sequence, stay for a while, and eventually move through giant stages before becoming white dwarfs Higher mass stars move rapidly off the main sequence and into the giant stages, eventually exploding in a supernova

Our Sun will eventually A. Become white dwarf B. Explode as a supernova C. Become a protostar D. Become a black hole

The spectral type of a star is most directly related to its a.Absolute magnitude b. Surface temperature c. Size or radius d. Luminocity

Which two vital parameters are used to describe the systematics of a group of stars in the HR diagram? a. Mass and weight b. Luminocity and radius c. Surface temperature and mass d. Luminocity and surface temperature