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The Stars

All the stars in the Big Dipper appear to be at the same distance from Earth, about 50 light-years away, but they are not. The table shows the names of the stars that make up the Big Dipper and the estimates of their actual distances from Earth.

How do we learn about stars when they are so far away?

Scientists use telescopes to view the brightness and light spectra of stars. Data from telescopes can be used to not only determine the distance of a star from Earth, but also the movement and the compositional elements of a star. The temperature of the star, for example, can be determined by looking at the star's color. Many stars are not white. They produce all colors of light but not at the same intensities. Stars that are very blue, producing large amounts of light at the violet end of the spectrum, are hot stars. Stars that are very red, producing large amounts of light at the red end of the spectrum, are cool stars.`

StarApproximate

Distance from Earth (light-years)

Dubhe 105

Merak 78

Phecda 90

Megrez 63

Alioth 68

Mizar 88

Alkaid 210

Humans have wondered about stars in the night sky for thousands of years. They named stars, grouped stars into constellations, and even created stories about how they came to be.

The picture on the right shows one of the most well-known star groups, the Big Dipper. The Big Dipper got its name because the stars appear to form the shape of a giant ladle.

The Big Dipper is actually part of the constellation Ursa Major, or the Great Bear. What do you see when you look at these stars: a ladle, the tail of a bear, or perhaps something else? Do you think the stars are all lined up, or are they actually at different distances from Earth?

Stars in the Big Dipper: Their Distances from Earth

A light-year is a unit used for measuring large distances

across space. It is the distance that light travels in one year, or

9.46 × 1012 km.

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The StarsHow do astronomers measure how far away a star is?There are two basic techniques for determining a star’s distance from Earth. The first method uses trigonometry and a phenomenon called parallax. Parallax is the apparent motion of an object according to the change in position of the observer. For example, hold your right index finger about 30 cm in front of your face. Close your right eye and observe your finger’s apparent position with respect to what is behind it. Now quickly close your left eye and open your right eye. Your finger should appear to move. The distance your finger seemed to move is a measure of its parallax.

Since Earth makes one revolution around the Sun every year, an astronomer can observe a star on two occasions that are six months apart (and that are a known distance apart since we know Earth’s orbital diameter) and measure the apparent angles and parallax of the star from Earth. The distance of the star can be calculated from this data in a process called triangulation. This process works well for stars that are about 400 light-years or closer to Earth.

This method, parallax, does not give accurate distances for stars greater than 400 light-years from Earth. Astronomers have to use other methods to determine the distances of these stars. Data collected from satellite-based telescopes indicated that certain stars, called cepheid variables, flicker like a candle. The speed of their flickering is due to how luminous they are. The more luminous they are, the more they flicker. If you can measure how long it takes for the star to flicker, you can determine very accurately the luminosity of the cepheid variable star. Why does the luminosity of a star matter? If you have a cepheid variable that is close, within 400 light-years, you can determine its distance using parallax. Comparing the star's brightness with its distance and luminosity gives you the relationship between those three variables. If another cepheid variable is found, measuring the brightness and luminosity (again using the rate of flickering) will tell you how far away the star is. The farther away from Earth a star is, the dimmer it appears to us. Think about how car headlights look dimmer the farther away they are from you, and how they appear brighter as they approach.

Parallax- the apparent shift of an observed object due to a change in the

position of the observer

The Hertzsprung-Russell Diagram shows the relationship between temperature and

absolute brightness or magnitude.

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The StarsHow do we know the movement of stars?

Scientists use telescopes to study the light spectra of stars to determine their movements. When stars move away from us, the wavelength of the light we observe from them appears to be shifted to the red end of the visible spectrum on the electromagnetic spectrum. The faster they are moving away, the farther the shift appears. When the stars are moving toward us, the wavelength of light is shifted toward the blue end. This shift of light, whether redshifted or blueshifted, is called the Doppler Effect. This phenomenon is also used to measure the distance to galaxies. This is is because all galaxies, except the closest ones, are all moving away from us. The farther away the galaxy is from us, the faster the galaxy is moving away from us.How do we know what stars are made of?

A star’s composition is also determined from a star’s light spectrum by a process called stellar spectroscopy. In this process, the light emitted from stars is split by a prism and separated into a spectrum of colors called an emission spectrograph. Every element has a unique spectrum of colors. Scientists compare a star’s spectrograph to spectrographs of elements to identify the chemical elements that make up the star.

Stars are furnaces that make the chemical elements found in the universe. In the first half of a star’s life cycle, it mostly converts hydrogen to helium. Most stars are thought to consist primarily of hydrogen and helium.

Stars convert hydrogen to helium using nuclear fusion in the core. After all of the hydrogen is used up in the core, the helium nuclei fuse to make heavier elements. Stars can form elements as heavy as iron and nickel as they mature into red giants. Scientists believe stars need the pressure and energy from a supernova to create elements heavier than iron and nickel.

A spectrograph of the star Eta Carinae.

The periodic table shows all the known chemical elements in the universe. Stars are the birthplace of the elements.

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The StarsStarlight of All Wavelengths

Stars do not emit just visible light. They emit light across the entire electromagnetic spectrum. The light spectrum ranges from radio waves, which are the size of buildings and cities, all the way down to gamma rays, which are smaller than atoms.

X-rays are generated by the hottest stars. They are also made by gas that is being heated as it is pulled into a black hole. The gas, squeezed and rubbing against each other, is so hot that it emits X-rays as it falls into the black hole. Gamma rays are made by the hottest objects in the universe: type II supernovae and hypernovae. Supernovae (and hypernovae) are generated by the death of the most massive stars. As the star collapses, the outer layers of the star are blasted into space. This releases so much energy that a massive amount of gamma rays are released. Some supernovae are so massive that the gamma rays contain more energy than the Sun will produce in its entire life.

While some telescopes are designed to collect visible light data, others are designed to collect data from other electromagnetic waves, such as radio waves and X-rays.

The kind of light emitted by a star is determined by its temperature. Radio signals are generated by cold objects, such as the gas found in nebular clouds in galaxies. Infrared light is made by living creatures and planets. Visible light is made by stars, as is ultraviolet light. Most of the ultraviolet light from the Sun is blocked by the ozone layer.

A black hole with its accretion disk of gas.

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The StarsTechnology for Discovering Space Objects

In 1608, Hans Lippershey, a Dutch eyeglass maker, invented a telescope that could magnify the field of vision by three times. Ever since, there have been great strides in advancing the technology of the telescope. Today telescopes positioned on Earth, in Earth’s orbit, and in space are used to collect information about objects within our solar system and beyond.

Telescopes collect waves of electromagnetic radiation, such as visible light. Astronomers use the collected data to learn about objects throughout the universe.

Data from visible light provide information about the movement of stars and galaxies through the study of redshift. The visible light spectrum of a star also provides information as to its chemical composition, density, luminosity, magnitude, and distance from Earth. Visible light telescopes are positioned on Earth, satellites, and space probes.

Data from radio waves provides information about the temperature of the universe as a whole, while data from X-rays, infrared rays, and gamma rays provide information about the hottest and coldest objects in space.

X-rays are produced by high energy events like the formation of a star or star system. X-ray telescopes can be positioned on Earth, but are commonly positioned on satellites.

Gamma rays are produced by extreme energy events, like black holes and exploding stars. Since Earth’s atmosphere blocks most gamma rays, telescopes for collecting gamma rays are typically positioned on satellites or on balloons.

Infrared radiation is produced by the universe’s coldest events, or events with the lowest amount of energy, and is used to find celestial bodies like asteroids, planets, and brown dwarf stars. Since dust is not visible in infrared light, infrared telescopes can be positioned on Earth’s surface.

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The StarsSatellites and Space Probes

The International Space Station (ISS) is an example of a satellite that orbits Earth. Scientists are able to perform experiments in a weightless environment and to collect data free from Earth’s atmosphere. Information is collected about Earth as well as other objects in our solar system and the universe. Satellite images indicate changes that have occurred on Earth’s surface and can track weather patterns.

The Hubble Space Telescope is one example of a data-collecting satellite in Earth’s orbit. Free from Earth’s atmosphere, the telescope collects light radiation from deep in space.

Satellites typically orbit Earth, but space probes travel can travel throughout the solar system and into space. Space probes have been used in missions to gather data about planets and other celestial objects by flying close. Various types of data, such as images, are collected and sent back to Earth.

Probes also can become satellites with long missions to orbit a single planet or body to gain detailed information. Radar may be used to create a map of the object.

Space probes can also be landed on a celestial body, such as a planet or asteroid, to gather surface and subsurface data. For example, surface probes have landed on Mars, the Moon, and on the asteroid 433 Eros.

Some asteroids contain valuable minerals. Missions to land on an asteroid and mine the minerals

are underway.

Satellite orbiting Earth

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The Stars

Do you think the age of a star can be determined by the chemical composition of its elements?

Try this:

The diagrams below represent various stages of one star in its star life cycle, showing the elements detected in its light spectrum at different times of its life cycle.

Based on this data, rank the stages of the star from the youngest (1) to oldest (5) stage:

H

H, He H, He, O

H, He, O, Si

H, He, O, Si, Fe, Ni

What can the composition or movement of a star tell us about how the universe was formed?

The big bang theory indicates the universe began with a period of rapid expansion. Calculations of the redshifts of stars throughout the universe indicate the galaxies are moving away from each other as the space between them expands. Spectral data of stars indicate the entire galaxy was originally composed of 75% hydrogen and 23% helium. This supports the idea of a common source of matter for the entire universe.