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Spectrum

In astronomy, the spectrum of an astronomical object is the rainbow of electromagnetic radiation emitted by the object, separated into its constituent wavelengths. A plot of the intensity of light at different frequencies. The band ranges in color from violet (shorter wavelength) to red (longer wavelength). A spectrum reveals a star's spectral type, radial velocity (from the spectrum's Doppler Shift), as well as the star's constituent elements. Plural = spectra.

A Star's Spectrum

A star's spectrum

Spectral Classification

Stars are classified by their spectra and their temperature. There are seven main types of stars. In order of decreasing temperature: O, B, A, F, G, K, and M.

O and B stars are very bright, but rare; M stars are common but dim.

For a detailed explanation of how stars are classified according to their spectral type and luminosity, please click here.

Dwarf Star

There are different classes of dwarf star. In general, however, a dwarf star is one with relatively low mass, small size, and average, or below average, luminosity. Our Sun is a yellow dwarf star.

Brown Dwarf

The star-making process is not always successful. A brown dwarf is star-like body that does not have sufficient mass to sustain the nuclear fusion needed to produce the radiant energy of a normal star.

It is thought that brown dwarfs form with enough mass to ignite nuclear fusion in their cores, but lack the extra mass required to make the fusion process self-sustaining.

It is believed that a body ten times as massive as Jupiter would be large enough to generate nuclear fusion, but it would it need an additional eight percent of the Sun's mass to make the process sustainable.

When its nuclear fusion ends, a brown dwarf still glows for a time from radiating heat, with a surface temperature around 2,500 Kelvin, or less.

Read about brown dwarf 2MASSJ22282889-431026 which has been studied using the Hubble and Spitzer space telescopes.

Brown Dwarf

Artist's illustration of brown
dwarf 2MASSJ22282889-431026

Main Sequence

Stars in this category maintain stable nuclear reactions and experience only small fluctuations in luminosity and temperature. Main sequence stars are considered to be in the stable, middle phase of their existence. They only move off the main sequence when the hydrogen fuel in their core becomes exhausted. At this point, a main sequence star, depending on its mass, will become a giant star, a supergiant, or a white dwarf. The more massive the star, the faster it consumes its nuclear fuel, and the shorter it remains in the main sequence.

Our Sun is a main sequence star.

Blue Supergiant

Blue supergiant stars are Class I supergiants of spectral type O. They are extremely hot and bright, with surface temperatures ranging between 20,000 and 50,000 degrees Celsius. The best known example is Rigel, the brightest star in the constellation of Orion. It has a mass of around 20 times that of the Sun and gives out more light than 60,000 suns added together. Despite their rarity and short lives, blue supergiant stars are heavily represented among the stars visible to the naked eye; their brightness makes them stand out.

Red Dwarf

A small, old, relatively cool star, approximately 100 times the mass of Jupiter. A typical red dwarf has a surface temperature of about 4,000 Kelvin, or less, and sometimes radiates as little as 1/10,000 of the Sun's light. Red dwarfs are the most common star type in the galaxy. Some known red dwarfs are Lacaille 8760, Lalande 21185, HD 179930 and HIP 12961.

White Dwarf

A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius about 0.01 times that of our Sun, but has a mass roughly equal to the Sun's. This makes a white dwarf about a million times denser than water!

As stars like our Sun near the end of their nuclear burning stage, they expel their outer layers, in the process creating planetary nebulae. The remaining cores of these stars become white dwarfs. Some of them are extremely hot, with temperatures excedeing 100,000 Kelvin.

Neutron Star

Neutron stars are created when giant stars, at the end of their life cycle, explode as supernovas and their cores collapse, with the protons and electrons essentially melting into each other to form neutrons. Only tens of kilometers in size, the smallest and densest stars known to exist, neutron stars have a mass about 1.4 times that of our Sun.

Currently, there are about 2,000 known neutron stars in the Milky Way and the neighbouring Megallanic Clouds.

Pulsar

First observed in 1967, pulsars are thought to be very rapidly rotating, small, and extremely dense neutron stars, the result of supernovae explosions. Pulsars are highly magnetized and emit regular pulses of electromagnetic radiation, especially radio waves. The radiation is focused into narrow beams by the stars' powerful magnetic fields. Since the magnetic poles of pulsars are not aligned with the poles of their rotational axes, the beams of radiation sweep around like a lighthouse beacon, and consequently are observed on Earth as short, regular pulses, with periods anywhere between one millisecond and 4 seconds.

The brightest pulsar so far discovered is located in the Cigar Galaxy [NGC 3034; M82].

Pulsar Schematic

Pulsar
Image credit: Wikipedia

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