Stars and Galaxies

Stars and Galaxies

Astronomers of Old• Aristotle-(384-322 BC) the great philosopher, proved that

the Earth is spherical, and believed that it was at the center of the universe. His reason for believing this was actually quite scientific: he knew that if the Earth revolved around the Sun, then we should see the stars shift position throughout the year. Since he did not have the technology to detect this shift, as we do today, he concluded that Earth must rest at the center of the universe. According to him, the Sun, planets, and stars were located in spheres that revolved around the Earth.

• Aristarchus (310-230 B.C., Greek) was the first to publish the idea that the Sun was actually in the center of the universe. His theory was considered far too radical. Unfortunately, history tends to forget that he came to this conclusion about 1,750 years before Copernicus did! He also attempted to measure the relative distances between the Earth and the Sun and the Earth and the Moon. Even though he used a reasonable method, his results were not very accurate, because he lacked the technological equipment to make a precise measurement.

• Hipparchus (190-120 B.C., Greek) is widely considered to be the greatest astronomer of ancient times. He compiled the first known star catalog to organize astronomical objects, and also came up with a scale to define the brightness of stars. A version of this magnitude system is still used today. He measured the distance from the Earth to the Moon to be 29.5 Earth diameters (we know today that the real value is 30 Earth diameters). Perhaps his greatest discovery was the precession, or wobble, of the Earth's axis, which is caused by the gravitational pull of the Sun and Moon.

Ptolemy- (about ad 100–170), astronomer and mathematician, whose synthesis of the geocentric theory—that the earth is the center of the universe—dominated astronomical thought until the 17th century.Copernicus -Nicholas Copernicus (1473 -1543) was a Polish astronomer, well known for his Copernican theory. His theory stated that sun rested near the center of the universe, and the earth, which spun daily on its axis, revolved annually around the sun. Now, this process is known as helocentric, or suncentered, systemGalileo -In 1609 he heard that in Holland a spy glass had been invented, and he was inspired to create the first telescope, which was as powerful as a modern day field glass. By December of the same year, he had built another telescope twenty times stronger than the first, which he was able to discovery craters on the moon with, stars in the milky way, and the four largest satellites of Jupiter.Newton- Newton made a huge impact on theoretical astronomy. He defined the laws of motion and universal gravitation which he used to predict precisely the motions of stars, and the planets around the sun. Using his discoveries in optics Newton constructed the first reflecting telescope.

Constellations• Make believe pictures in the sky

– Named for mythological characters or animals

– Used to find locations for stars

• Ursa Major-Big Dipper• Ursa Minor-Little Dipper

– Polaris: Last star in Little Dipper’s handle

Canis Major-Big Dog

Sirius-Dog’s shoulder

Orion-Hunter with lion

Betelgeuse-Orion’s shoulder

Ursa Major as a Guide

Orion as a Guide

Circumpolar constellations are visible all year round because they circle around Polaris (the north star. The are so close to Polaris that they never set below the horizon.

Ursa Major

Ursa Minor

Casseopeia

Cepheus

Draco

NORTHERN CIRCUMPOLAR CONSTELLATIONS

Magnitude

• Magnitude –The brightness of a star

– Absolute Magnitude- the measure of the actual brightness of a star or the light it gives off

– Apparent Magnitude- The amount of light that reaches the earth

A bright star may appear dim if it is far away

The apparent magnitude scale is a “reverse scale”; that is, the lower the number, the brighter the apparent magnitude

The 26 Brightest Stars

A relatively dim star might appear very bright if it is closer

For instance, Sirius looks much brighter than Rigel in the sky. Is it really??? Sirius is REALLY much closer to Earth than Rigel. If they were the same distance from earth, Rigel would appear much brighter

The 26 Nearest Stars

Experience apparent magnitude when you are riding in a car at night. Observe the headlights as an approaching car gets closer.

Parallax

• Parallax is the apparent shift of the position of an object when viewed from 2 different positions

• The distance to a fairly close star can be measured by measuring the parallax with respect to the background stars

• Parallax

• A light year is a unit of measure that is used for stars that are farther away.

• It is equal to 300,000 m/s or 9.5 trillion km/y• The nearest star other than the sun is Proxima Centauri, at 4.2 light

years away (about 40 trillion km)

Spectra

• The color of a star indicates its temperature. Blue white stars are hot. Red or orange stars are cooler

• The spectra of a star is its visible light broken down into colors (wavelengths)– As light radiated from a star passes through its atmosphere,

some of it is absorbed by elements– Where these elements are present, there is an absence of color,

or a black line in the spectra– The black lines in the spectra indicate the elements in the stars

atmosphere

THE SUN• 99% of all the matter in the solar system is in the sun. It sustains life on earth, but in

space, it is just another star.• The sun is a middle sized, middle aged star with average magnitude that shines with

a yellow light• Like other stars, it is a huge ball of gas which produces energy by fusing hydrogen

into helium in its core• Energy produced by the fusion in the core travels outward by radiation and convection.• Layers of the sun:

– Photosphere-lowest layer and layer that gives off light. Called “surface of the sun”. Temp: 6000K

– Chromosphere-above photosphere. Extends 2000 km above photosphere. Above that is a transition zone (2000 km to 10,000 km)

– Corona-above transition zone. Largest layer of sun’s atmosphere. Extends millions of km into space. Temp: 2 million K

The sun is unusual in that it is NOT in a binary system. Most stars occur in pairs and orbiteach other. Many times, stars are in groups, called star clusters. They are gravitationallyattracted to each other.NASA/Marshall Solar Physics

Sunspots• Sunspots: Areas of the sun that appear dark

because they are cooler than surrounding areas. – Galileo first identified sunspots. We learned from

them that the sun rotates (sunspots appear to move) – The sun rotates faster at the equator than at its poles.

The sunspots at the equator take 27 days to make a rotation; 31 days at the poles.

– Sunspot Maximums-times when there are more sunspots than at other times. These occur every 11 years

Solar Flares and Prominences

• Solar prominences- caused by the sun’s intense magnetic field– huge arching columns of gas– blast material from the sun into space at speeds of 60 km/s to 1000 km/sChromospheric FeaturesSolar Flares- when gases near a sunspot brighten up suddenly and shoot gasoutward at high speed.– -UV light and X-rays from flares reach earth and cause disruptions of

radio signals. Radio and telephone communications can be difficult because of solar flares. High energy particles emitted by flares are captured by the Earth’s magnetic field and disrupt communications equipment

– -These particles also interact with the Earth’s atmosphere near the polar regions and create lights called “aurora”. In the north, it is called “Aurora Borealis”; in the south, it is called “Aurora Australis”

Flares, Spots and Prominences

Evolution of StarsThe H-R Diagram-Two scientists in the early 1900’s noticed a direct

relationship between the magnitude of stars and their temperatures. They developed a graph to shop this relationship

The temperatures of the stars in Kelvins go across the X axis and the increasing magnitude goes up the Y axis. Most stars fit in a diagonal band from upper left to lower right. The following links show a diagram, and how stars evolve.

The Hertzsprung-Russell DiagramHR Diagram SimulatorThe diagonal band of stars is called the “main sequence”. It contains

hot, blue stars in the upper left and cooler, red, dim stars in the lower right. The medium stars such as our sun are in the middle. (Yellow, medium-temperature, medium-brightness). About 90% of all stars are main sequence stars and most cluster in the lower right. Of the remaining 10%, some are hot but not bright and located in the lower left (white dwarfs). Others are bright but not hot and are in the upper right. These are called giants or red giants or supergiants

Sun’s EnergyPeople have long wondered what could cause the

sun (and other stars) to shine

In 1920, a scientist hypothesized that temperatures within the sun must be hot (duh). Another scientist said that with these high temperatures, hydrogen could fuse into helium and tremendous amounts of energy would be released. Fusion occurs in the cores of stars because only there is the temperature and pressure high enough to cause atoms to fuse.

Life of a StarCradle to the grave

I. Stars begin as a large cloud of gas called a nebula(1)a. Particles of gas and dust exert a gravitational force of each other and contract. b. This causes the nebula to be unstable and it fragments into smaller pieces. c. Each of these collapses to form a protostar(2). This is the stage in the evolution of a young star after it has fragmented from a interstellar gas cloud but before it has collapsed sufficiently for nuclear fusion reactions to begin. It might last 100,000-10 million years.d. Temperatures increase to 10 million K, fusion begins, energy radiates outward through the condensing ball of gas into space, and a star is born

II. The new star becomes a main sequence star (3)a. The pressure of the heat from the fusion going on inside balances with the gravitational attraction. This stage may last for 10 billion years

Newborn stars are forming in the Eagle Nebula. This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust. The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars. At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed. The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.

The Orion Nebula

III. Giants (4)

a. After the star has burned up all its hydrogen, there is no longer a balance between the inside and outside, so the core contracts and heat inside increases.

b. The core is hot enough for the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a Giant.

c. In about 5 billion years, our sun will be a giant.

d. When its helium supply is used up, the outer layers escape into space and all that’s left is a hot, dense core which contracts under gravity. It is now…

IV. A White Dwarf (5)(and when it stops shining — a Black Dwarf

V. Supergiants(5b) and Supernovas(6b)a. Massive stars evolve in a similar way as small stars except the stages happen in a more quick and dramatic way. b. Heavier elements form in the core and the star expands into a Supergiantc. Iron starts to form in the core and when this happens fusion can no longer occur. d. The core collapses and violently sends a shockwave outward, the outer portion explodes and a Supernova forms which can be billions of times brighter than the original star

SN2004dj supernova has the brightness of 200 million suns

Neutron Stars and Black Holes• The collapsed core of a supernova shrinks to 10-15 km

in diameter and becomes a neutron star. The protons and electrons combine to form neutrons (neutron star)

• Neutron Stars are so dense that a teaspoonful of the matter in one would weigh TONS.

• Neutron Stars and Pulsars - Introduction• If the remaining core is more than 2 X more massive

than the sun, it evolves into a black hole, an object so dense that NOTHING can escape its gravity field!!

• Black Holes-If you shined a flashlight on a black hole, the light wouldn’t shine—it would just disappear.

• Scientists locate black holes by looking for X-rays around them. The matter pulled into black holes collides with other material and generates X-rays

The Big Bang• Big Bang Theory states that about 15 billion years ago, the universe

began with a huge explosion.• The Big Bang Theory

– We can see photos taken with satellites telescopes like Hubble that are light years in the past (since they are light years away) and see how the universe looked back then. These galaxies are in various stages of development and may be from when the universe was no more than 1 billion years old.

• Explosion about 15 billion years ago• Within fractions of a second the universe grew from the size of a

pen point to 2000 times the size of the sun• When it was a second old, it was a dense opaque mass of

elementary particles• After about 300,000 years, matter collected and formed into

galaxies. As matter cooled, hydrogen and helium gas formed. • More than 1 billion years after the initial bang, the first stars were

born and light shone from them

GALAXIES• A galaxy is a large group of stars, gas and dust held together by

gravity. The Milky Way contains about 200 billion stars including the sun. Galaxies

• Galaxies are group together into clusters. • The Milky Way is part of the Local Group which contains about 30

galaxies of various types and sizes. • The three types of galaxies are Elliptical, Spiral and Irregular. The

Milky Way is a normal spiral galaxy and its spiral arms are composed of stars and gas and radiate out from an area of densely packed stars called the nucleus. It is about 100,000 light years across and the sun is about 30,000 light years from its center. The sun orbits around the center of the Milky Way once every 240,000 million years

• The Andromeda Galaxy is in our Local Group and is about 2.2 million light years away. The Andromeda Galaxy appears as a fuzzy blur in the constellation.

• Elliptical galaxies are the most common type. They are shaped like large three-D ellipses—football-shaped or round. Irregular galaxies are irregular shapes. Two of these are the Clouds of Magellan.

Doppler Shift

• We know the Universe is expanding because we can see evidence of stars moving away from each other. This is known as the Doppler shift. We see the wavelengths of light stretched apart when stars move away from us. This is called a “red shift” because the dark lines in the spectrum shift toward the red end. A blue shift happens when a star moves away from us and the black lines move toward the blue end of the spectrum

The schematic diagram below shows a galactic star at the bottom left with its spectrum on the bottom right. The spectrum shows the dark absorption lines first seen by Fraunhofer. These lines can be used to identify the chemical elements in distant stars, but they also tell us the radial velocity. The other three spectra and pictures from bottom to top show a nearby galaxy, a medium distance galaxy, and a distant galaxy. The pictures on the left are negatives, of course, so the brightest parts of the galaxies are black. Notice how the pattern of absorption lines shifts to the red as the galaxies get fainter. The numbers above and below the spectra are the measured wavelengths in nm (nanometers). In the star which is at rest with respect to us, or in a laboratory standard, the line wavelengths are 393 & 397 nm from Ca II [ionized calcium]; 410, 434, 486 & 656 nm from H I [atomic hydrogen]; 518 nm from Mg I [neutral magnesium]; and 589 nm from Na I [neutral sodium]. By measuring the amount of the shift to the red, we can determine that the bright galaxy is moving away at 3,000 km/sec, which is 1 percent of the speed of light, because its lines are shifted in wavelength by 1 percent to the red.