Stars, Galaxies & the universe

Stars are huge, hot, bright balls of gas that are trillions of kilometers away from Earth.

Stars are made up of different elements in the form of gases. The inner layers of a star are very dense and hot. The outer layers of a star, or a star’s atmosphere, are made up of cool gases.

Constellations are patterns of stars in the sky. As Earth revolves around the sun, the apparent locations of the constellations change from season to season.

When you look at white light through a glass prism, you see a rainbow of colors called a spectrum. Spectrum – the band of color produced when white light passes through a prism. The spectrum consists of millions of colors, including red, orange, yellow, green, blue, indigo and violet.

The spectrum of a star is different. A star’s spectrum is made of dark emission lines. Emission lines are lines that are made when certain wavelengths of light, or colors, are given off by hot gases.

When an element emits light, only some colors in the spectrum show up, while all the other colors are missing. Each element has a unique set of bright emission lines. This spectrum scale is called an electromagnetic spectrum.

The electromagnetic spectrum includes the entire range of radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Often times, scientists will place telescopes onto satellites, such as the Hubble Telescope in order to collect information about stars.

Astronomers use spectrographs to get information about stars. Spectrographs break the light from an object into colors and photograph the resulting spectrum. Then, they can gain information on the chemical compositions and temperatures of stars.

The 3 main characteristics used to classify stars are: Size a. Neutron star (smallest star) b. White dwarf star c. Medium- sized star d. Giant star e. Supergiant star (largest star) Temperature ☼ A star’s color reveals its temperature. Brightness ☼ Apparent magnitude – a star’s brightness as seen from Earth. ☼ Absolute magnitude – the brightness a star would have if it were at a standard distance from Earth.

Stars vary greatly in size Stars vary greatly in size. Giant stars are very large stars; typically 10 to 100 times larger than the sun and more than 1,000 times the size of a white dwarf.

Temperature differences between stars result in color differences that you can see. For example, stars with a surface temperature above 30,000*C are blue in color.

The brightness of a light or star as seen from Earth is called apparent magnitude. The brightness that a star would have at a distance of 32.6 light-years from Earth is called absolute magnitude.

Because stars are so far away, astronomers use light-years to measure the distances from Earth to them. A light-year is the distance that light travels in one year; about 9.5 trillion kilometers.

Stars near Earth seem to move, while more-distant stars seem to stay in one place as Earth revolves around the sun. A star’s apparent shift in position is called parallax. The shift can be seen only through telescopes. Astronomers use parallax and simple trigonometry to find the actual distance to stars that are close to Earth.

Because of Earth’s rotation, the sun appears to move across the sky. Likewise, if you look at the night sky long enough, the stars also appear to move. This is called the apparent motion of stars.

The apparent motion of the sun and stars in our sky is due to Earth’s rotation. Each star is moving in space. However, their actual motion is hard to see because they are so distant. If you could put thousands of years into one hour, a star’s movement would be obvious. This is known as the actual motion of stars.

A star enters the first stage of its life cycle as a ball of gas and dust. Gravity pulls the gas and dust together into a sphere. As the sphere becomes denser, it gets hotter and the hydrogen changes to helium in a process called nuclear fusion. This stage is also known as the stellar nebula.

As stars get older, they lose some of their material As stars get older, they lose some of their material. This can happen slowly or quickly. Much of the material of the star is returned to space. Some of the material combines with more gas and dust to form new stars.

There are four main types of stars 1. main-sequence stars 2. giants 3. supergiants 4. white dwarfs A star can be classified as one type of star early in its life cycle and then can be classified as another star when it gets older.

Main-Sequence Stars – 2nd stage of life. During this stage, energy is generated in the core of the star as hydrogen atoms fuse into helium atoms. This process releases an enormous amount of energy. The size will change very little.

Red Giants – 3rd stage of life. The star expands and cools once it uses all of its hydrogen. It can be 10 or more times bigger than the sun. Eventually, the center of the star will shrink. As the center shrinks, the atmosphere of the star grows very large and cools to form a red supergiant.

Supergiants – 4th stage of life. The star is at least 100 times bigger than the sun.

White Dwarfs – 5th and final stage of life. It is the size of the sun or smaller. It is a small, hot star that is the leftover center of an older star. It has no hydrogen and can no longer generate energy by fusing hydrogen atoms into helium atoms. They can shine for billions of years before they cool completely.

Astronomers use a Hertzsprung-Russell Diagram, better known as an H-R diagram to understand how stars change over time. Most of the stars in the H-R diagram form a diagonal line called the main sequence.

Stars that are more massive than the sun may explode with such intensity that they become a variety of strange objects such as supernovas, planetary nebulas, neutron stars, pulsars, and black holes.

Supernova – a gigantic explosion in which a massive star collapses and throws its outer layers into space. They are massive blue stars. The explosion is so powerful that it can be brighter than an entire galaxy for several days.

Planetary nebulas – a usually compact luminous ring-shaped nebula that is composed of matter which has been ejected from a hot star at its center.

Neutron star – a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons.

Pulsars – rapidly spinning neutron stars that emit rapid pulses of radio and optical energy. They send out beams of radiation that spin very rapidly. The beams are detected by radio telescopes as rapid clicks or pulses.

Black holes – objects so massive and dense that even light cannot escape their gravity. They do not gobble up other stars as seen on TV. They are difficult to locate because there is no light. If a star is nearby, some gas or dust from the star will spiral into the black hole and give off X rays that allow astronomers to detect it.

Galaxies are classified into three main categories: 1. Spiral galaxies – the shape of twin spirals 2. Elliptical galaxies – looks like flattened spheres 3. Irregular galaxies – no regular shape

Spiral Galaxies Have a bulge at the center and spiral arms. The spiral arms are made up of gas, dust and new stars that have formed in these denser regions of gas and dust. Examples are the Andromeda Galaxy and the Milky Way.

Elliptical Galaxies Have very bright centers and very little dust and gas. Contain mostly old stars. Few stars form here because there is no free-flowing gas. Example – M87 galaxy

Irregular Galaxies These galaxies do not fit in any other category. The shape is irregular. Many of these galaxies are close companions of large spiral galaxies. Example – Large Magellanic Cloud

Galaxies are composed of billions of stars and some planetary systems, too. Some of these stars form gas clouds and star clusters.

Nebula – a large cloud of dust and gas in interstellar space; a region in space where stars are born or where stars explode at the end of their lives.

Globular cluster – a tight group of stars that looks like a ball and contains up to 1 million stars.

Open cluster – a group of stars that are close together relative to surrounding stars.

Looking at distant galaxies reveals what early galaxies looked like. This information gives scientists an idea of how galaxies change over time and may give them insight about what caused the galaxies to form.

Quasars – very luminous, starlike objects that generate energy at a high rate; quasars are thought to be the most distant objects in the universe. They are also one of the most powerful energy sources in the universe. Some scientists think that quasars may be the core of young galaxies that are in the process of forming.