Section 1: The Life and Death of Stars

Section 2: The Milky Way and Other Galaxies

Section 3: Origin of the Universe

Key Terms

Star

Light-Year (ly)

Red Giant

White Dwarf

Supernova

Black Hole

What are Stars?

Stars are huge spheres of very hot gas that emits light and other radiation

The nearest star to Earth is the sun

Since ancient times, we have learned that stars are located at different distances from Earth. We use the unit light-year (ly) to describe a star’s distance from Earth.

1 ly is the distance that light travels in one 9.5 x 1015m

Stars are driven by nuclear fusion reactions

Stars are huge spheres of very hot hydrogen and helium gas that emits light. They are held together by the enormous gravitational forces that result from its own mass.

The pressure is more than a billion times the atmospheric pressure of Earth. The temperature is hotter than 15 million Kelvin, and the density is more than 13 times the density of lead.

Nuclear fusion take s place in the core

Fusion combines the nuclei of hydrogen atoms into helium. When two particles fuse energy is released. The energy creates outward pressure that balances the inward pull of gravity.

Energy moves slowly through the layers of a star. The energy moves through the layers of a star by a combination of radiation and convection.

Convection - rising hot gas moves upward, away from the star’s center, and cooler, denser gas sinks towards the center.

Radiation - energy is transferred to individual atoms. The

atoms absorb the energy and then transfer it to other atoms in random directions. Atoms near the surface radiate energy into space.

Energy from nuclear fusion may take millions of years to

work its way through a star Once light leaves the surface of a star, it radiates across

space at the speed of light in a vacuum 3 x 108m/s. It takes light from the sun about 8 minutes to reach Earth.

Studying Stars Why do some stars appear brighter than others? The brightness of a star depends on the star’s

temperature, size, and distance from Earth The brightest star in the night sky is Sirius. Sirius

appears so bright because it is relatively close to Earth, only about 9 ly away.

We learn about stars by studying light

(Electromagnetic Spectrum).

A star’s color is related to its temperature

When light from a glowing hot object passes through a prism, it generates a spectrum of many colors

Hotter objects glow with light that is more intense and that has short wavelengths (blue end of the spectrum) cooler objects have greater intensity and longer wavelengths (closer to red).

Spectral lines reveal the composition of stars

Because each element produces a unique pattern of spectral lines astronomers can match the dark lines in star light to the known lines of elements found on Earth.

The sun’s mass is 71% hydrogen, 27% helium, and 2% other

elements The Fate of Stars Stars begin forming from clouds of gas and dust called

nebula. Stars are born, go through different stages of development,

and eventually die. Stars appear different because they are in different stages of life.

Nearly 90% of all stars in our galaxy, including the sun, are in midlife.

The sun will become a red giant before it dies A red giant is a large reddish star late in its life cycle.

The star is red because the surface is cooler, but the core is hot enough to convert helium into carbon and oxygen

Once the helium is gone the core of a red giant will

contract causing the outer layers to expand.

The result will be a white dwarf, a small and very dense star about the size of Earth.

Stars with a mass of 1.4 solar masses and smaller will

have a similar life cycle. Most stars in our galaxy will end as a white dwarf.

Supergiant stars explode in supernovas Massive stars evolve faster than smaller stars do.

They also develop hotter cores that create heavier elements through fusion.

Forming an iron core signals the beginning of a Supergiant star’s violent death.

Why is this the beginning of a violent death?

Because fusing iron atoms to make heavier elements requires adding energy rather than releasing energy.

A Supernova is a gigantic explosion in which a massive star collapses and throws its outer layers into space.

After a supernova either a neutron star or black hole will form

If the core that remains after a supernova has a mass

of 1.4 to 3 solar masses, the remnant can become a neutron star. Only a few kilometers but very massive.

A thimbleful of a neutron star would weigh more

than 100 million tons on Earth We can detect neutron stars by pulsars or sources of

pulsating radio waves

If the left over core is greater than 3 solar masses it will collapse forming a black hole

A black hole is an object so massive and dense that not even light can escape its gravity.

They can be detected indirectly by observing the radiation of light and X-rays from objects that revolve rapidly around them.

The H-R diagram shows how stars evolve

The vertical axis indicates brightness. Absolute magnitude indicates how bright stars would be if they were all the same distance from Earth. The horizontal axis indicates surface temperature of the stars, with hotter temperatures on the left.

When stars are born, they appear as protostars on a diagonal line called the main sequence.

The position of a star on the main sequence depends on the initial mass of the star. A stars position on the H-R diagram changes during its life cycle.

Red giants are both cool and bright so they appear in the upper right. White dwarf stars are both faint and hot, so they appear in the lower left.

Our sun went from a protostar to a main sequence star in tens of millions of years. It will remain here for 10 billion years. After this it will move to the upper right of the H-R diagram, becoming a Red giant. It remains here for about 100 million years. Then it will become a white dwarf, in the lower left, about 50 million years later.

Key Terms

Galaxy

Cluster

Interstellar Matter

Quasar

Galaxies While stars are a few light-years away, the nearest

galaxy to ours is millions of light-years from Earth A galaxy is a collection of millions or billions of stars.

There may be more than 100 billion galaxies. If you counted 1000 galaxies per night, it would take 275,000 years to count all of them.

Galaxies, such as the Andromeda Galaxy, contain

millions to billions of stars bound together by gravity

Young stars are often found near the nebular gas and dust where they where born. Older stars may be throughout the galaxy or in regions that contain no gas and dust.

Gravity holds galaxies together in clusters

Without gravity, everything in space might be a veil of gas spread out through space. Galaxies are not spread out evenly through space. They are grouped together in clusters. The members of a cluster of galaxies are bound together by gravity.

The Milky Way galaxy and Andromeda galaxy are two of the largest members of the Local Group, an cluster of more than 30 galaxies.

A cluster of galaxies can form even larger groups called superclusters.

A typical supercluster contains thousands of galaxies containing trillions of stars. They can be as large as 100 million ly across and are the largest structures in the universe.

Types of Galaxies

Edwin Hubble divided all galaxies into three

major types: spiral, elliptical, and irregular.

Spiral - have spiral arms made up of gas,

dust and stars

Elliptical - have little gas or dust

Irregular - have no particular shape

We live in the Milky Way Galaxy

Most of the objects we see in the night sky are part of the Milky Way. Because our solar system is inside the Milky Way, we cannot see all of it at once. Our solar system is located within a spiral arm, about 26,000ly from the center.

The Milky Way is a spiral galaxy

Our galaxy, like most spiral galaxies, is a huge spiraling disk of stars, dust, and dust with a huge bulge in the center.

The nucleus of the galaxy is very dense and has many old stars. The gas and dust have been used up to form stars. At the center may be a large black hole.

Spiral galaxies, such as Messier 74 (M74), have gas and dust between the stars called interstellar matter. Clouds of interstellar matter provide materials that allow new stars to form.

Spiral arms are often blue because of hot young stars and the bulge is often red because of old stars.

Elliptical galaxies have no spiral arms and are spherical or egg shaped. They contain mostly older stars and have little interstellar matter.

They are found in a wide range of sizes, from Giant elliptical galaxies containing trillions of stars and up to 200,000 ly in diameter to dwarf elliptical galaxies containing a few million stars.

Because of their shape, it is difficult to determine the positioning of elliptical galaxies in relation to Earth.

All other galaxies are irregular galaxies Edwin Hubble named the third category irregular

galaxies because they lack regular shape and do not have a well-defined structure.

Some irregular galaxies contain little interstellar

matter while others have large amounts and contain mostly young blue stars.

The Megallanic Cloud is a large irregular galaxy that is easily seen in the Southern Hemisphere and is part of the Local Group of galaxies.

Irregular galaxies may be oddly shaped because the gravitational influence of nearby galaxies distorts their spiral arms

How Galaxies Evolve When scientist study distant galaxies, they are

looking back in time. If we study a galaxy that is one billion ly away, we are observing light that left that galaxy one billion years ago.

Quasars may be infant galaxies In 1960, a faint object was finally matched with a

strong radio signal. This was the first quasar, or quars-stellar object.

Quasars are very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe

Galaxies change over time All stars change over time. Massive stars explode in

supernovae, and lower-mass stars become red giants and , eventually, white dwarfs. Because galaxies are made up of gas, dust, and stars that change over time, galaxies will also change.

Galaxies can also change when two galaxies approach

each other because of gravity.

Key Terms

Universe

Red Shift

Blue Shift

Big Bang Theory

What is the Universe?

The universe is the sum of all space, matter, and energy the exist, that has existed in the past and that will exist in the future.

The solar system is only a small part of our galaxy, which is but one of many galaxies in one of many cluster of galaxies.

We see the universe now as it was in the past

A year is a unit of time and a light-year is a unit of distant

We never see light from distant objects as it really is, right now. When we see very distant objects, we see them as they were when they were younger or when the light left.

We can compare how galaxies age by looking at many galaxies at different distances, and therefore at different ages.

It is extremely hot facing the sun in space, and very cold facing away from it. Space is a vacuum with no air and no air pressure.

What Happened at the Beginning?

The universe is expanding. In 1929, Hubble found that spectral lines from other galaxies almost always shift towards the red end of the spectrum. This is the red shift.

Red Shift is an apparent shift toward longer wavelengths of light caused when a luminous object moves away from the observer.

When an object is approaching us, the shift is toward

shorter wavelengths of the spectrum’s blue end. Blue Shift is an apparent shift toward shorter

wavelengths of light caused when a luminous object moves towards the observer.

Expansion implies that the universe was once smaller.

Since the universe appears to be expanding rapidly outward like an explosion, scientist call this hypothetical explosion the big bang.

If the expansion has been constant since the big bang, we can estimate the age of the universe. Velocity is equal to distance divided by time.

Using estimates for distance and velocity, scientist estimate that the age of the universe is between 10-20 billion years.

Did the universe start with a big bang? The Big Bang Theory states that all matter and energy

in the universe was compressed into an extremely small volume that 10-20 billion years ago exploded and began expanding in all directions

Cosmic background radiation supports the big bang

theory Cosmic Background Radiation is a very dim signal all

over the sky in the form of radiation at microwave wavelengths

The universe has an overall temperature of about 2.7K

Processes in stars lead to bigger atoms Hydrogen fuels stars and acts as a building block for

other elements. All elements other than hydrogen and helium form in

stars. Nuclear fusion in stars produces helium and elements up to the atomic number of iron. Heavier elements form during supernovae.

Predicting the Future of the Universe The future of the universe is uncertain The universe is still expanding, but it may not do so

forever. There are three possibilities for the outcome of the

universe

1. The universe will keep expanding forever 2. The expansion of the universe will gradually slow down, and the

universe will approach a limit in size 3. The universe will stop expanding and start to fall back in on itself

The fate of the universe depends on mass If there is not enough mass, the gravitational force

will be two weak to stop the expansion, so the universe will keep expanding forever. If there is just the right amount of mass, the expansion will continually slow down but will never stop completely. If there is more mass then this right amount, gravity will eventually overcome the expansion an the universe will start to contract leading to the “big crunch”

New technology helps scientist test theories

There is debate about dark matter and what it is Astronomers estimate the mass of the universe by

measuring stars, galaxies, and matter in the interstellar medium. However, there is still matter that is undetectable

Scientist call this undetectable matter dark matter. Dark matter may be planets, black holes, or brown

dwarfs. It may even be exotic atomic particles that no one knows how to observe

Scientist use mathematics to build better models to help them study the universe

According to Einstein’s theory of relativity mass curves space, much in the same way that your body curves a mattress when you sit on it.