Stellar Classification

How we know• We learn about stars by

looking at them through spectroscopes.

• All stars produce a spectra that tells us about their composition.

• The dark lines on this spectra are called absorption lines.

• They tell us what elements are present in that star.

What is a spectrum?

• This is the visible light spectrum like a rainbow.• This is wavelength of light that our human eyes can

see.• Each color represents a different wavelength.

• Every element when burned will produce a spectrum called a bright line spectrum.

• Or more commonly called an emission spectrum. Like these 2.

• Stars produce the same bright line spectrum for every element they have in them.

• BUT because the star light has to pass through the vastness of space and our atmosphere we see dark lines instead.

Spectral types of stars reveal their chemical compositions and temps.

• Stars are classified into spectral types–divisions of the spectral classes• O, B, A, F, G, K, and M

– Subclasses of each type• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

• The original letter classifications originated from the late 1800s and early 1900s

2 new classes

• Brown dwarfs are now placed in 2 new classes called L and T.– Unlike true stars, brown dwarfs are too small to

sustain thermonuclear fusion.

Full Spectral TypingSpectral Class and Luminosity Class

• The Sun– Classified as a G2 V

• Luminosity classes (use Roman numerals)– I – Giant– II – Giant– III – Giant– IV – Sub-giant– V – Main Sequence

Relationship between a star’s luminosity, radius, and surface temperature

• Stars come in a wide variety of sizes and temperatures.

• The energy output from the surface per second is called a stars luminosity.

The Hertzsprung-Russell (H-R) Diagram• The H-R diagram is a graph

plotting the absolute magnitudes of stars against their spectral types—or, equivalently, their luminosities against surface temperatures

• The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature

Morgan-Keenan Luminosity classes