Unit 5: Astronomy

Unit 5: Astronomy

Section 2: Stars (Part 1)

No one knows just how many stars exist in the universe, but the number is undoubtedly in the hundreds of trillions. A star is a massive sphere of hydrogen and helium gas that emits energy and is held together by gravity. Our Sun is the source of almost all the energy used on Earth by living organisms. For thousands of years, humans have looked to the stars with wonder and worship, trying to understand them. They imagined constellations, or groupings of stars in the night sky that can be formed into a recognizable pattern. Stars have been used to navigate ships, predict the changing of the seasons, and gave rise to the zodiac. Only recently have scientists began to discover the true nature of stars. Remember stars do not disappear during the day, they simply get out-shined by our sun!

The Formation of a Star (E-8)

Stars form in the center of nebulas that are beginning to collapse towards the center. As more matter gathers in the center of a nebula, it becomes more concentrated. Matter under that kind of pressure becomes hotter and denser. When the temperature reaches about 15 million degrees Celsius, hydrogen atoms in the gas combine or fuse to create helium atoms. This process is called nuclear fusion and that reaction continues until the star runs out of hydrogen to convert.

Fusion reactions inside a star create very high amounts of pressure, and like a bomb stronger than any nuclear bomb, threaten to blow the star apart from the inside. However, the star remains intact because it is in a state of equilibrium. The gravity of the star pulls on each part of it and keeps the star together as it radiates energy out in all directions, providing solar energy to the surrounding orbiting bodies. Sometimes the birth of new stars is triggered by one galaxy colliding or passing through another, mingling enough clouds of dust and gas to get the process started.

Classifying Stars (E 73)

Stars are classified according to color, temperature, size, composition, and brightness.

Astronomers use a magnitude scale to describe the brightness of objects they see in the sky. A star’s brightness decreases with the square of the distance. Magnitude is a better measurement than just visual brightness because one star might look brighter than another simply because it is closer.

Absolute magnitude – amount of light it gives off.

Apparent magnitude – amount of light that is received by observers here on Earth. Planets can have an apparent magnitude as well which has to do with their reflectivity or albedo.

Astronomers study stars with spectrographs (an instrument for analyzing the color of light an object is giving off) to determine their temperature and chemical composition. Different elements, when heated, emit different colored light and this allows us to understand its composition. There are 7 main categories of stars separated by their temperatures.

Luminosity is a term used by astronomers for the total rate at which a star emits radiation energy. Unlike apparent brightness, luminosity does not depend on how far away the star is. All stars are sorted by their temperature into one of the categories of luminosity. It is called the Hertzsprung-Russell (HR) diagram in honor of the astronomers who discovered it in the early 1900s.

The Lifecycle of a Star (E 74)

When nuclear fusion begins in a star it is considered to be a “main-sequence” star. Most stars spend 90% of their lifetimes on the main sequence. Newborn stars grow outward and in doing so bathe the cloud of gases and dust (called a molecular cloud) in strong ultraviolet radiation. This vaporizes the cloud, creating beautiful sculpted shapes.

How long a star lives depends on its mass. Stars like our Sun will live about 10 billion years. Smaller, cooler stars might go on twice that long, slowly burning their fuel. Massive supergiant stars consume their mass much more quickly, living for only tens of millions of years. However, in the end, all stars will burn out eventually.

Throughout its life a star loses mass in the form of a stellar wind, or solar wind in the case of our Sun. As the star ages it loses more and more mass. Stars the size of our Sun and smaller will end their days as tiny, shrunken remnants of their former selves, surrounded by beautiful shells of gas and dust. In about 5 billion years, our Sun will burn out. Scientists have studied dead stars, so we already know the fate of our solar system.

The Death of Supergiants (E 77)

Massive stars (supergiants tens of times more massive than our Sun) also lose mass as they age, but at some point their cores collapse catastrophically. The end of a supergiant’s life is a cataclysmic explosion called a supernova. In an instant of time, most of the star’s mass is hurled out into space, leaving behind only tiny remnant called a neutron star. If the star is massive enough, the force of the explosion can be so strong that the remnant is imploded into a stellar black hole. Black holes are still somewhat of a mystery, however what we do know is that the gravity is so strong that not even light can escape it.

Over the billions of years, stars have been created and destroyed from the same recycled matter. The material that is given off by dying stars migrates around space and Earth was formed from these materials. In other words, the matter that makes up who you are came from that same source.

Unit 5: Astronomy

Section 2 Glossary of Terms

Constellation – a grouping of stars in the night sky into a recognizable pattern.

Nuclear Fusion – a nuclear process that releases energy when lightweight nuclei combine to form heavier nuclei.

Solar Wind – a flow of hot charged particles leaving the Sun.

Albedo – the reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%.

Spectroscopy – the science that studies the way light interacts with matter.

Spectroscope – an instrument consisting of, at a minimum, a slit and grating (or prism) which produces a spectrum for visual observation.

Luminosity – the total amount of energy radiated by an object every second.

Molecular Cloud – a large, cold cloud made up mostly of molecular hydrogen and helium, but with some other gases, too, like carbon monoxide. It is in these clouds that new stars are born.

Supernova – the death explosion of a massive star whose core has completely burned out. Supernova explosions can temporarily outshine a galaxy.

Neutron Star – the imploded core of a massive star produced by a supernova explosion.

Stellar Black Hole – the leftover core of a massive single star after a supernova. Black holes exert such a large gravitational pull that not even light can escape.

Section 2 Questions (Part 1)

16. The two main gases that make up a star are ______and ______.

17. Give two examples of how our ancestors used the stars.
______

18. When matter is put under extremely intense pressure it becomes more ______and ______.

19. When hydrogen atoms combine to form a helium atom, a free neutron, and pure energy, this process is called ______.

20. Stars do not explode because they are held together by ______.

21. Astronomers use a ______scale to describe the brightness of objects.

22. How many categories of stars are there? ______

23. What is the hottest color a star can have? ______What is the coolest? ______

24. When an object in space reflects light from a star it is called ______.

25. What is the name of the diagram that measures the luminosity of stars? ______

26. Stars spend 90% of their lifetimes in a phase called ______

27. Do smaller or larger stars live longer? ______Why?
______

28. About how old is our Sun? ______
About how much longer will it survive? ______

29. When large stars go supernova, they can become either ______or ______.

30. Black holes are so strong that not even ______can escape its gravity.

Unit 5: Astronomy

Section 2: Stars (Part 2)

The Structure of a Star (E 50)
Like the Earth, stars have a layered structure. Its central region, the core, is where nuclear fusion occurs. The core is the source of all the energy the star emits. That energy travels out from the core, through a radiating layer and a convection zone above that. Finally, it reaches the outer layers: the photosphere, the chromosphere, and the corona. The photosphere is the star’s visible surface. The chromosphere produces much of the star’s ultraviolet radiation. The corona is the superheated uppermost layer of the star’s atmosphere.

The Sun and the Earth’s Atmosphere (E 50)

The Sun is the Earth’s main external energy source. Of all the incoming energy from the Sun, about half is absorbed by the Earth’s surface. The rest is either

- absorbed by the atmosphere.

- reflected or scattered back into space by the Earth or clouds.

Molecules of dust and gas in our atmosphere interfere with some of the incoming solar radiation by changing its direction. This is called scattering, and it explains the blue color of the sky. Reds and oranges are seen when the sun is close to the horizon and must travel through a thicker part of the atmosphere.

The Sun heats the atmosphere indirectly by warming the surface of the planet which then radiates the heat outward in the form of infrared radiation. The average albedo of our planet is about 0.3, which means we reflect about 30% of the light we receive from the sun. However, snow covered areas tend to reflect more while dark soil or forested areas reflect less.

The Electromagnetic Spectrum (E 63)

In 1666, Sir Isaac Newton found that he could separate light into a range of colors from red to violet if he directed a beam of sunlight through a glass prism. He concluded from this experiment that sunlight is actually a mixture of all the visible colors of the rainbow and perhaps some light we cannot see. About 10 years later, a scientist by the name of Christiaan Huygens proposed the idea that light travels in waves. Visible red has the longest wavelength and violet has the shortest. All forms of electromagnetic waves travel at the speed of light.

Stars radiate energy over a very wide range of wavelengths. Earth’s atmosphere shields you from some of the most dangerous forms of electromagnetic radiation such as ultraviolet, gamma rays, and x-rays. You know by now that ultraviolet (or UV radiation) causes sunburn by damaging your exposed skin cells and that a gas called ozone protects you from it. Ozone is made up of three oxygen atoms combined. However, pollution from cars and other sources is destroying this natural supply of ozone and exposing Earth to dangerous levels of UV radiation.

Humans can only see wavelengths between 0.4 and 0.7 µm, which is why it is called the visible spectrum. A micrometer (µm) is a millionth of a meter. Infrared radiation is felt as heat, although it is not visible. Microwaves can be concentrated and used to heat food, while radio waves can be used to communicate, and x-rays are used to see bones.

Astronomy and the Electromagnetic Spectrum (E 64)

Although humans can study stellar objects like the moon and perhaps Mars rather easily, learning about stellar objects like the Sun is much more challenging. Studies like these require special instruments, especially for measuring wavelengths outside of the visible spectrum.

Radio telescopes – produce images of celestial bodies by recording different amounts of radio emission coming from an area of the sky observed. Multiple dish antennas are built and more than 100 images might be combined for a single complete image.

X-ray telescopes – can detect highly energetic radiation streaming from objects like supernova explosions, active galaxies, and black holes.

Infrared telescopes – instruments sensitive to radiation being produced by star-forming nebulae and cool stars. Example: The Hubble Space Telescope

Sunspots and Solar Flares (E 53)

Sunspots are small dark areas on the Sun’s visible surface. They can be as small as Earth or as large as Uranus or Neptune. They are formed when magnetic field lines just below the Sun’s surface are twisted and poke through the solar photosphere. They look dark because they are about 1500 K cooler than the surrounding surface and they also extremely magnetic. Sunspots can last for a few hours to a few months and tend to correlate in frequency with solar flares.

Solar flares occur when an enormous amount of ultraviolet, x-ray, and radio waves blast out from the Sun. In addition, protons and electrons stream from flares at 800 km/hr. The flow of charged particles (also called plasma) create intense solar winds. These high radiation events can be devastating to Earth’s electrical systems on the ground as well as damage or interfere with satellites. Sometimes called “space weather,” the increased amount of radiation can harm astronauts or people traveling in planes over polar regions where the atmosphere is the thinnest.

At least one effect of space weather is quite wonderful. When the solar wind encounters the Earth’s magnetic field, it excites the gases in the Earth’s atmosphere, causing them to glow. The charged particles concentrate in an oval shaped area near the Earth’s magnetic poles. The result is a beautiful display called the aurora. They appear as shimmering curtains of white, green, or red lights in the sky.

Unit 5: Astronomy

Section 2 (Part 2) Glossary of Terms