Basic Properties of Stars Astronomy

Kirchhoff’s Three Kinds of Spectra

A Model of a Hydrogen Atom

Spectral Lines A. Electrons have a definite binding energy. B. Each element has its own set of energy levels C. If an electron absorbs enough energy, it jumps to a higher energy level. D. When an electron falls, it releases energy in the form of light. E. wavelength inversely proportional to frequency F. Dark lines are produced when a cooler gas absorbs light. G. An emission spectrum shows the chemical element that produced those lines.

Electron Distances and Energy Levels

Possible Absorption and Emission Lines for the Hydrogen Atom

An Emission Spectrum of Hydrogen

Stellar Spectra A. Predominantly patterns of dark lines on a continuous band of colors. B. Star’s bright visible surface is called the photosphere. C. As light travels through the star’s outer atmosphere, the cooler gases absorb some colors/wavelengths.

Chemical Composition A. Our sun was the first absorption spectrum analyzed in 1814 by Fraunhofer 1. Fraunhofer lines--strongest dark lines from the sun B. By comparing the dark lines with spectral lines from other elements, we find what’s in the sun.

Spectral Classes A. Absorption spectra are used to classify stars into 7 types. B. If hydrogen lines are stronger… 1. It’s not because of more hydrogen…ALL stars have hydrogen. Stars are classified in the following order: O, B, A, F, G, K, M “oh, be a fine girl/guy, kiss me !”

Spectral Classes E. So what’s the difference? 1. Stars at different temperatures display certain lines better than others. The temperature is the difference ! class O stars are hottest….class M stars are coolest.

The Spectra of Radiation Emitted with Temperatures of 4500 K, 6000 K, and 7500 K Things will become “bluer” when they are hotter. Stars will become “redder” when they are cooler. If we can find the brightest part of the spectrum of a star, we can find its temperature.

Temperature B. Every chemical element has a characteristic temperature and density at which its most effective in producing certain lines. C. At extremely high temps.--Helium atoms are ionized; bluer stars (class O) D. Temps. Around 5800 K--metal atoms E. Temps. Below 3500 K--titanium oxide molecules; redder stars (class M)

Spectra of the Spectral Classes

The Relative Number of Hydrogen Atoms in the Second Energy Level for Various Temperatures

The Number of Hydrogen Atoms with Their Electrons in the Second Energy Level Compared with the Total Amount of Hydrogen, Whether in Atomic or Ionized Form

The Relative Numbers of Atoms of Different Elements on a Typical Star

Other information from Spectral Lines A. Other info is gathered from spectral lines. B. Collisional broadening--broader lines might show a denser star C. Rotational broadening--broader lines can show how fast a star rotates/spins D. Zeeman effect--split lines show magnetic fields E. Redshift--lines shifted toward the red show a star moving away (blueshift—means star is moving towards you)

The Spectra of a Rapidly Rotating Star and a Slowly Rotating Star

The Doppler Shifts of a Rotating Star

The Parallax of a Nearby Star A parsec is a unit of distance such that a star that exhibits a shift of 1” (1 second or 1/3600 of a degree) of arc. This is only an apparent shift of the star in the sky (and it’s very small) as a result of the real motion of the earth around the sun. We are looking at the star at different angles. The distance of a star can be found by observing its parallax angle. The equation is: distance (in pc) = 1 / parallax angle(“) Example: Alpha Centauri has a parallax angle of 0.742”. So its distance from Earth is 1/0.742” = 1.35 parsecs. To convert this to light years (1pc = 3.26 ly): 1.35 pc x 3.26ly = 4.4 ly

The Proper Motions of a Nearby and Distant Stars

Propagation of light Remember that light falls off according to the inverse square law An object 3x farther away will appear 1/32 = 1/9 as bright

Apparent magnitude (m) Definition: a measure of how bright a star appears The general rule: the lower the number, the brighter it appears

Apparent magnitude (m) The modern magnitude scale is set up so that a difference in magnitudes goes up as an exponential function 2.512(x) Where x is the difference in apparent magnitudes of A and B

Absolute magnitude (M) Definition: a measure of how much light a star is putting out into space (its luminosity) The general rule: the lower the number, the more luminous it is Note: you can’t just say, “that star is brighter”…do you mean it appears brighter, or do you mean that it’s giving off more light? Question: Why would it matter? Answer: a really luminous star might appear fainter simply because it’s very, very far away

Absolute magnitude (M)