Stars and Their Characteristics Stars and Galaxies LYRA HERCULES CYGNUS DRACO BOÖTES CANES VENATICI URSA MINOR CEPHUS LACERTA COMA BERENICES CASSIOPEIA PEGASUS URSA MAJOR CAMELOPARDALIS LEO PERSEUS PISCES LEO MINOR LYNX VOCABULARY constellation Hydrogen and helium are the two most abundant elements in stars. apparent magnitude astronomical unit light-year Stars can be grouped into constellations. parsec luminosity absolute magnitude
Stars and Their Characteristics Betelgeuse There are stars of different brightness in the constellation Orion, including two of the brightest stars as viewed from Earth—Betelgeuse and Rigel. Rigel Stars and Galaxies Stars differ in mass, size, and surface temperature. Surface temperature affects the color of stars. Apparent magnitude, luminosity, and absolute magnitude are used to describe the brightness of stars.
Stars and Their Characteristics Stars that show variation in brightness are known as variable stars. Distances in space are measured in astronomical units, light-years, and parsecs. • Light-Year The distance that light travels in one year, about 9.5 trillion kilometers. • Parsec A unit of measurement used to describe distances between celestial objects, equal to 3.258 light-years.
Apparent Magnitude Some stars appear very bright but are actually fainter stars that lie closer to us. Similarly, we can see stars that appear to be faint, but are intrinsically very bright ones lying far away from Earth.
Apparent Magnitude Apparent Magnitude The measure of how bright a star appears to be to an observer on Earth.
Stars and Their Characteristics Luminosity:The brightness of a star or the power radiated by the star. The luminosity is a quantity that depends on the star itself, not on how far away it is. For this reason a star's luminosity tells you about the internal physics of the star and is a more important quantity than the apparent brightness.
What does the luminosity of a star depend on? • Temperature (proportional to T4) • Size (proportional to R2) • Full blown formula? L=4pR2sT4
A star can be luminous because it is hot or it is large (or both!). The luminosity of an object = the amount of energy every square meter produces multiplied by its surface area.
Caution! • Do not confuse the size of an object with the mass of an object. • Just because an object is large in dimension does not necessarily mean it is also large in mass. • For example, you can have a forty foot tall by three foot across marshmallow that looks “large,” but that does not mass as much as that of a “small” football sized hunk of lead.
Absolute Magnitude Absolute Magnitude:The measure of how bright a star would be if it were located 10 parsecs from Earth.
On the left-hand map of Canis Major, dot sizes indicate stars' apparent magnitudes; the dots match the brightnesses of the stars as we see them. The right-hand version indicates the same stars' absolute magnitudes — how bright they would appear if they were all placed at the same distance (32.6 light-years) from Earth. Absolute magnitude is a measure of true stellar luminosity.
Inverse Square Law • As the light from a star goes into space it fills a larger and larger spheres. • The area of a sphere is given by its radius: A = 4 p d2 • d is the radius of the sphere The amount of light we receive from a star decreases with the square of our distance from the star: Amount of light = L0 / d2 Flux=“amount of light”
Measuring the Distance to Stars • Measuring distances is difficult. The best method for measuring distances of nearby stars is called parallax. It relies on observing a star from two different places.
Measuring the Distance to Stars Measuring the Parallax Angle: The parallax angle p is illustrated in the following figure.
Measuring the Distance to Stars Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the apparent shift of an object against a background caused by a change in observer position.
The Distance to the Stars • We obtain a different perspective on a star by observing it at different times of the year. • In 6 months the Earth has moved 2 AU away. • (2AU = 300 million km) • The parallax method lets us measure the distance to stars about 1000 light years away.
Measuring Distances: Parallax • The larger the star’s distance, d, the smaller its parallax p. • So distance and parallax are inversely related. d = 1 / p
Measuring Distances: Parallax • Most stars have a parallax angle, p, which is very small. • The angle of parallax, p, is usually measured in arc seconds • 60 arc seconds = 1 arc minute • 60 arc minutes = 1 degree. • Distances to stars are measured in either: light years, or parsecs. • 1 parsec = 3.2 light years • If a star’s parallax is 1 arc second, then its distance is 1 parsec. (parsec = PARallax of one arcSEC)
Parallax Examples • If a star’s parallax is 1 arc second its distance is 1 parsec • Question: If a star has a parallax of 0.1 arc seconds what is its distance in parsecs? Answer: d = 1 / p d = 1/ (0.1) = 10 parsecs = 3.2 light years
Constellations: A group of stars that appear to form a pattern in the sky.
Constellations • Constellations are easily recognizable patterns that help people orient themselves using the night sky. There are 88 “official” constellations. • Hundreds of the brightest stars, those visible with the unaided eye, were given names in ancient times. • Today stars are named by their coordinates on the celestial sphere. This is an imaginary sphere surrounding Earth.
Constellations All stars and objects in space, can be mapped relative to the poles and equator of the celestial sphere. Their position north or south of the celestial equator — essentially their latitude — is called “declination.” Their position east or west essentially is their longitude, or right ascension, measured in hours, minutes, and seconds.
Constellations The stars are distant objects. Their distances vary, but they are all very far away. Excluding our Sun, the nearest star, Proxima Centauri, is more than 4 light years away. As Earth spins, the stars appear to move across our night sky from east to west, for the same reason that our Sun appears to “rise” in the east and “set” in the west.
Constellations • If observed through the year, the constellations shift gradually to the west. This is caused by Earth’s orbit around our Sun. In the summer, viewers are looking in a different direction in space at night than they are during the winter.
Constellations • Stars close to the celestial poles, the imaginary points where Earth’s north and south axes point in space, have a very small circle of spin. Polaris, Earth’s north “pole star,” will appear to move very little in the night sky. The farther from Polaris, the wider the circle the stars trace.
Constellations • Stars that make a full circle around a celestial pole, like those in the Big and Little Dippers in the northern hemisphere, are called “circumpolar stars.” They stay in the night sky and do not set. At the equator, there are no circumpolar stars because the celestial poles are located at the horizon. All stars observed at the equator rise in the east and set in the west.
LYRA HERCULES CYGNUS DRACO BOÖTES CANES VENATICI URSA MINOR CEPHUS LACERTA COMA BERENICES CASSIOPEIA PEGASUS URSA MAJOR CAMELOPARDALIS LEO PERSEUS PISCES LEO MINOR LYNX Constellations
Life Cycles of Stars The Hertzsprung-Russell diagram plots a star’s luminosity against its surface temperature. The diagram’s groups of stars represent life-cycle stages of stars. Most stars are main-sequence stars. Red Supergiants Blue Supergiants Highest <Main Sequence> Red Giants Luminosity White Dwarfs Red Dwarfs Lowest Hertzsprung-Russell diagram Temperature Hottest Coolest Stars and Galaxies VOCABULARY main sequence giant star supergiants white dwarf nebula planetary nebula supernova neutron star pulsar black hole
Hertzsprung-Russell Diagram: • Ejmar Hertzsprung (1873-1967) – Copenhagen – Began his career as a Chemical Engineer. While working and independently at the same time… • Henry Norris Russell (1877-1957) – Princeton – Student then professor. • A graph that separates the effects of temperature and surface area on stellar luminosities. • The HR Diagram is much like the same thing as producing a graph of people’s height vs. weight.
Life Cycles of Stars Stars and Galaxies A star’s fate depends on its mass. A star with a mass similar to the sun’s will become a white dwarf.
Life Cycles of Stars Stars and Galaxies A star with a mass eight or more times greater than the sun’s will either become a black hole or a neutron star.
Properties of stars • Color and temperature • Hot star • Temperature above 30,000 K • Emits short-wavelength light • Appears blue • Cool star • Temperature less than 3000 K • Emits longer-wavelength light • Appears red
Properties of stars • Color and temperature • Between 5000 and 6000 K • Stars appear yellow • e.g., Sun • Binary stars and stellar mass • Binary stars • Two stars orbiting one another • Stars are held together by mutual gravitation • Both orbit around a common center of mass
Hertzsprung-Russell diagram • Shows the relation between stellar • Brightness (absolute magnitude) and • Temperature • Diagram is made by plotting (graphing) each star's • Luminosity (brightness) and • Temperature
Hertzsprung-Russell diagram • Parts of an H-R diagram • Main-sequence stars • 90% of all stars • Band through the center of the H-R diagram • Sun is in the main-sequence • Giants (or red giants) • Very luminous • Large • Upper-right on the H-R diagram
Hertzsprung-Russell diagram • Parts of an H-R diagram • Giants (or red giants) • Very large giants are called supergiants • Only a few percent of all stars • White dwarfs • Fainter than main-sequence stars • Small (approximate the size of Earth) • Lower-central area on the H-R diagram • Not all are white in color • Perhaps 10% of all stars
Birth of a Star: Nebula • Stars are born in a glowing cloud of interstellar gas and dust (mostly hydrogen), called a nebula. • Gravity causes every atom and every bit of dust to pull on every other one and all move to the center, causing the protostar to collapse. • Because the atoms move faster and faster as they fall toward the center, friction is created as they rub together and the temperature rises.
Birth of a Star: Nebula • Heat causes the protostar to glow in with its own light, giving off even more light than our Sun even though it is not nearly as hot. • When a temperature of about 27,000,000°F is reached, nuclear fusion begins. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the star. • The protostar is now a stable main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die.
Birth of a Star: Nebula Black Widow Nebula
Birth of a Star: Nebula Crab Nebula
Main Sequence Stars Main Sequence: A star that is at the point in its life cycle in which it is actively fusing hydrogen nuclei into helium nuclei; Our sun is a main sequence star.
Giant Stars A Giant Star: is large star with great luminosity and a diameter 10 to 100 times greater than that of the sun. A giant star is one of two kinds very large stars the other being a Red giant or Supergiant Red giants are stars of 1000 times the volume of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.
Supergiants Supergiants: are the most luminous, most massive stars, with diameters greater than 100 times the diameter of the sun. 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.
White Dwarf :The remnant of a giant star that has lost its outer atmosphere; the glowing stellar core. • A white dwarf is what stars like our Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula. White Dwarf: Sirius–B, this white dwarf is very hot due to high density and rapid spin.