A star is a celestial body of gases that radiates energy derived from fusion reactions in its interior.

The energy from the fusion reactions in the interior of a star is transported to the surface of the star. Let's follow the particles of energy - photons - generated in the core from the interior to the surface of the star. First, a photon travels from the fusion core to the next layer in a star called the radiative zone where photons carry energy to the next layer called the convection zone. In the convection zone, gas is heated by the outbound photons and the hot gas is carried to the visible surface of the star which is called the photosphere.

The visible surface of a star is called the photosphere. The temperature of the photosphere depends on the mass of the star and age. Surface temperatures vary over thousands of degrees from star to star. Our sun has a surface temperature of 6,000 C, whereas the surface temperature of Rigel is nearly 30,000 C. That's quite a difference. Temperatures aren't the only differences among stars. We also observe a large range in luminosity - the amount of energy emitted per unit time. The luminosity also depends on the star's mass and age. So, it is not surprising that we find a correlation between luminosity and surface temperature. The table below shows masses, tempertures, and luminosities for some typical stars. Note how more massive stars are more luminous and hotter. This correlation is seen in the Hertzsprung-Russell (HR) Diagram, built primarily from spectroscopic observations. The HR Diagram was first described between 1912 and 1914 independently by Hertzsprung and Russell.

To understand spectroscopy, we must first discuss the wave nature of light. Light is Nature's way of transporting energy through space. Waves carry energy. So we talk about light as a wave. The characteristics of lightwaves include a length of the wave (wavelength), how many lightwaves pass your eye per second (frequency), and speed - in this case the speed of light or 300,000 km/sec. Waves of visible light range from about 350 nanometers (nm) to about 600 nm. We perceive 350 nm lightwaves as the color violet, and 600 nm lightwaves as the color red. You can see the other colors between violet and red as shown below by putting a prism between you and a white light source.

Odd thing, though - if you hold a prism up to a red neon sign rather than white light, you will only see a series of orange or red bands, not the entire rainbow of light from violet to red. Or, if you hold a prism between your eye and a sodium streetlight, you will see only a few yellow bands. Each element you look at has its own fingerprints of colors. Some examples are shown below. How does this work? Physicists at the turn of the 20th century asked the same question. The answer led to models of the atom and an understanding of the interaction between energy and matter.

Neon Spectrum
Mercury Spectrum
Hydrogen Spectrum
Air Spectrum

Put a prism in front of a telescope and point the telescope at the stars. You will see the spectra of the stars!

The spectrum of a hot star shows primarily hydrogen absorption lines. The spectrum of a cooler star shows absorption lines of heavier elements.
For example, the spectrum of the star shown below has strong hydrogen absorption lines which means it is a hot star.

The spectrum of another star shown below has very weak hydrogen absorption lines, with other absorption lines due to elements like sodium and iron, and even molecules like titanium oxide.

The spectrum of a star is the result of absorption in the outer visible surface of the star. The outer visible surface of a star consists of gases which absorb the continuum of light being emitted from the interior of the star. The resulting spectrum is observed as an aborption spectrum. The observed absorption spectrum depends on the composition of gases and surface temperature of the star.

The image below shows stars in a 5 degree by 5 degree area of the sky. A prism has been set in front of the telescope when the image was taken. Rather than seeing the star images on the image, you see that each star appears as its spectrum. Each elongated streak is actually the spectrum of a star! Notice that many stars that have similar spectra.

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These major classifications are subdivided even further. The O type stars are subdivided into O5, O6, O7, O8, and O9. The B, A, F, G, and M classes of stars are subdivided into 10 smaller units: B0 to B9; A0 to A9; F0 to F9; G0 to G9; and M0 to M9. The K class of stars is subdivided into 8 smaller units: K0 through K7.

Astronomers also noticed that any particular subclass can be generally either a supergiant, giant, or dwarf star - indicating the amount of luminosity, or size, of the star. These luminosity classes are given the Roman Numerals I, III, and V for Supergiant, Giant, and Dwarf stars.

Putting all this together, for example, our Sun is a G2V star, which means it is a cooler (6,000 C) dwarf star.

The spectral classifications for several stars are given in the table below, along with the temperature and luminosity of each star.

Star Name	Spectral Type	Mass (in Solar Masses)	Temperature (C)	Luminosity (in Solar Luminosities)
Theta Ori	O6 V	40	40,000	210,000
Alpha Cru	B1 V	13	26,000	16,000
Regulus	B7 V	3.5	10,300	150
Vega	A0 V	2.1	9,600	37
Altair	A7 V	1.7	8,000	10.7
Sun	G2 V	1	6,000	1
Epsilon Eri	K2 V	0.8	3,900	0.3
Kapteyn's Star	M1 V	0.38	3,800	0.004
Barnard's Star	M4 V	0.16	3,100	0.0004

Millions of stars have yet to be classified, and some of these don't fit any class! Be the first to discover the spectrum and nature a star!

Brief history of stellar spectral classification: Harvard College Observatory Director Edward Pickering began a program in 1886 to obtain spectra of stars. In 1896, Annie Jump Cannon joined the team at HCO identifying the spectra of stars. The classification scheme used the letters A to O sequentially where A stars have the strongest and clearest absorption lines. Cecilia Helena Payne-Gaposchkin demonstrated that the spectral sequence O B A F G K M is actually a sequence in temperature in her thesis "Stellar Atmospheres, A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars". In 1925 she became the first person to earn a Ph.D. in astronomy from Harvard. Astronomer Otto Struve characterized it as "undoubtedly the most brilliant Ph.D. thesis ever written in astronomy". A mnemonic invented by one of PARI's students to remember the order goes like this: “Our best astronauts finally got kelvin measurements” We use this same classification scheme today. The spectral order of the most common classifications are shown above.

You will be classifying stars from their spectra which have been scanned from photographs of the sky. The photographs cover an area of the sky of 1.25 degrees by 5 degrees, so there are many star spectra on each image, most of which have never been explored before!