A star is a ball of gas in space giving off tremendous amounts of electromagnetic energy. This energy comes from nuclear fusion. If you look closely at stars, you will see they are not just specks of white light, but rather vary somewhat in color. Astronomers analyze light the stars emit by using a spectrograph and are able to determine what gases make up the star and how hot it is. Different elements absorb different wavelengths of light.
The surface temperature of a star is indicated by its color. Blue stars shine with the hottest temperatures and Red stars with the coolest. Most star temperatures are in the range from 2800 C – 24,000 C, although some can be even hotter.
Blue stars generally have an average surface temperature of 35,000 C (63,000 F). Red stars are the coolest, with an average surface temperature of 3000 C (5400 F). Yellow stars, like our Sun, have an average surface temperature of 5500 C (9900 F).
Stars are separated into 7 spectral classes based on the star’s temperature (thus, its color). Those spectral classes are as follows: Oh Be A Fine Girl/Guy, Kiss Me! Every class is divided into a subclass indicating tenths of the range between two classes. Ex: A7 is 7/10 of the way between class A and class F.
Two kinds of motion are associated with stars: 1. APPARENT MOTION • Earth’s rotation causes the illusion of stars moving around a central star, Polaris, commonly known as the North Star. • Earth’s revolution around the Sun causes stars to be visible during different seasons.
2. ACTUAL MOTION • First, they move slightly across the sky (only see the closest ones). • Second, they may revolve around another star (binary system). • Third, they may either move away from or toward our solar system. • The apparent shift in the wavelength of light emitted by a light source moving away from or toward an observer is called the Doppler Effect. • Also used for sound waves on Earth.
Since space is enormous, distances between stars and Earth are measured in light- years, the distance light travels in a year. ▪ About 9.46 trillion km. (63,000 AU) ▪ Light from the Sun takes about 8 minutes to reach us. For closer stars, astronomers determine a star’s distance by measuring parallax, the apparent shift in a star’s position when viewed from different locations.
Astronomers use two scales to describe a star’s brightness: Apparent Magnitude: the brightness of a star as it appears from Earth. (App. Mag. Sun = -26.8) ▪ Depends on both how much light the star emits and how far it is from Earth. ▪ The lower the number on the scale, the brighter the star appears to us. Absolute Magnitude: the actual brightness of a star, assuming all stars were set at a standard distance from Earth. (Abs. Mag. Sun = 4.8) ▪ Found by in-depth calculations setting all stars an equal distance from the observer(s).
The Hertzsprung-Russell Diagram, or HR Diagram, is a chart showing the surface temperature and absolute magnitude (brightness) of stars. Named for Ejnar Hertzsprung and Henry Norris Russell. Highest temperatures are plotted on the left and the brightest stars at the top. Most stars fall in a diagonal band extending from cool, dim red stars at the lower right to hot, bright blue stars at the upper left known as the main sequence. Stars on this band are called either main sequence stars or dwarf stars. The Sun is a main sequence star.
The luminosity class is added in Roman numerals after the temperature spectral class. Indicates the size of the star. ▪ I – Supergiants ▪ II – Bright Giants ▪ III – Giants ▪ IV – Subgiants ▪ V – Dwarfs (Main Sequence Stars) ▪ VI – Subdwarfs ▪ VII – White Dwarfs The Sun is a G2V star.
A star begins in a nebula, which is a cloud of gas and dust in space. Consists of about 70% Hydrogen (H2), 28% Helium (He), and 2% of other elements. Particles are sometimes pulled together by gravity and dense regions of matter build up within a nebula.
As gravity makes these dense regions more compact, it begins to spin. Flattening of the disk will begin and the central concentration of matter is now called a protostar. ▪ Continues to contract and increase in temperature for millions of years. ▪ Once temperature reaches about 10,000,000 C (18,000,000 F) nuclear fusion begins. ▪ Marks the birth, or the first stage, of a star and nuclear fusion can continue for billions of years.
The second and longest stage in the life of a star is the main- sequence stage. Energy continues to be generated at the core of a star as Hydrogen (H2) fuses into Helium (He). A star with a mass close to the Sun’s may stay on the main-sequence for about 10 billion years.
A star enters the third stage when 20% of the Hydrogen (H2) atoms within its core have fused into Helium (He) atoms. Helium (He) core of the star contracts and begins fusing into Carbon (C) and at high enough temperatures, into Oxygen (O2). Temperature increases in the Helium (He) core and energy will be transferred to the star’s outer shell, causing expansion. This shell of gases grows cooler (turns red) as it expands and these large red stars are known as giants.
Main-sequence stars more massive than the Sun will become larger than giants in their third stage and will be extremely bright and are known as supergiants. Often times 100 times larger than the Sun.
The remains of low-mass and medium-mass stars collapse into a hot, extremely dense core of matter known as a white dwarf. Shine for billions of years before they cool completely. Hot, but dim due to their size (about the size of Earth). They become fainter until they cool completely and are known as a black dwarf.
Stars with masses more than 8 times the mass of the Sun can undergo massive explosions known as supernovas. Once stars use up their fuel supply, the core collapses and energy is released and the outer layers explode.
After a star explodes into a supernova, the core may contract into a very small, yet incredibly dense ball of neutrons, called a neutron star. A single teaspoon of neutron star material would have a mass of 2 x 1030 kg. Rotate very rapidly. Neutron stars emitting a beam of radio waves and spinning very rapidly are known as pulsars. Detected by radio telescopes.
Some massive stars produce leftovers too massive to become stable neutron stars. Further contraction takes place under greater gravity and this crushes the dense core of the star and leaves a black hole. Gravity is so great, not even light can escape. Locating them is rather difficult. X-rays are released as matter is pulled into a black hole.
Because stars are so distant from us on Earth, they appear in different patterns. One of 88 regions into which the sky has been divided in order to describe the locations of celestial objects are known as constellations.
A large-scale group of stars, gas, and dust bound together by gravity is called a galaxy. A typical galaxy has approximately 200 billion stars. Astronomers estimate the universe contains hundreds of billions of galaxies. 3 types of galaxies:
A nucleus of bright stars with flattened arms spiraling around the nucleus. The spiral arms consist of billions of young stars, gas, and dust. Our Milky Way galaxy is an example of a spiral galaxy.
Galaxies varying in shape from nearly spherical to very elongated (elliptical). Extremely bright in the center and have NO spiral arms. Consist of very few young stars and small amounts of interstellar material (gas and dust).
No particular shape. Usually have low total masses and are rich in interstellar material. Make up only a small percentage of the total number of observed galaxies.
Although cosmologists, astronomers who study the universe, have proposed several different theories to explain the expansion of the universe, the current and most widely accepted is the Big Bang Theory. States that billions of years ago, all the matter and energy in the universe was compressed into an extremely small volume approx. 15 BYA, an explosion sent all the matter outward and is currently still expanding.