Starry NightVincent Van Gogh

The StarsCh 14 The Sciences

• Astronomy, the study of objects in the heavens, was probably the first “science”.

• Astronomy is the scientific study of cosmic objects and celestial happenings.

• Please do not call it astrology.

Great Idea:

The Sun and other stars use nuclear fusion reactions to convert mass into energy.

Eventually, when a star’s nuclear fuel is depleted, it must burn out.

The stars that we see throughout the year vary.

Common constellations

• Big dipper

• Southern cross

…distinguish between a star and a planet.

At most, we see 3000 stars

“light year”

• A unit of distance

• One light year equals about ten trillion kilometers (9.5 x 1012 km)

• or about 6 trillion miles,

• the distance light travels in a year.

• (speed of light = 300,000 km/sec)

Optical Telescopes: Reflecting Refracting

Large Reflecting Telescopes

• Keck telescopes, Mauna Kea, Hawaii

• European Southern Observatory, Chile

Radio Telescopes

Figure 14-4

Schematic diagrams of telescopes. In an optical telescope (a), light strikes a curved mirror and is focused on a light-sensitive detector such as the eye or a piece of film. In a radio telescope (b), radio waves from space strike a curved metal dish that focuses the waves onto an antenna. Signals are amplified and processed by computer.

Hubble Space Telescope

• launched in 1990 by the space shuttle Discovery

• Oops! Mis-shaped mirror; repaired in ’93, ’97, ’98.

• Now it’s taking great pictures of far away galaxies

• Latest images!

Next Generation Space Telescope

• NGST and NASA’s Origins Project

• Evolution of galaxies• Production of

elements by stars• Process of star and

planet formation

The Sun Gives Off Energy

• Nuclear fusion reactions

• hydrogen atoms merge to form helium

The Sun: a nuclear furnace

• Sun produces energy at an enormous rate, equivalent to the explosion of 92 billion nuclear bombs every second

• 700 million tons of hydrogen turn into helium every second near the center of the Sun, with 5 million tons vanishing, turned into pure energy

Why is the Sun the size it is?

The size of the sun…

• Gravity pulls in, pressure pushes out, and the Sun is the size it is:

1.4 million km in diameter or over 100X the diameter of the Earth.

 The Parts of the Sun

Inside the Sun

• Core

• Convection Zone

Outer Region of Sun

• Photosphere

• Chromosphere

• Corona

 The Parts of the Sun

Figure 14-2

The magnetic field of the Earth is swept out into a long tail by the solar wind.

aurora borealis or “northern lights”

Stars “live” and “die”.

“life cycle” of a Sun-like star

1) The star is created when gas and dust condense, forming a “young stellar object”

All stars have a beginning and an ending

Young stellar objects (YSOs)

• Newborn stars that are still surrounded by their birth clouds.

• Example: Orion Nebula

Orion Nebula

“life cycle” of a Sun-like star

2) Shrinking, the star dispels its birth cloud, and its hydrogen fire ignites and nuclear fusion is underway.

3) As the hydrogen burns steadily, the star joins the main sequence stage of stellar life.

Figure 14-6

A Hertzsprung-Russell diagram plots a star’s temperature versus its energy output.Stars in the hydrogen-burning stage, including the Sun, lie along the main sequence, while red giants and white dwarfs represent subsequent stages of stellar life.

“life cycle” of a Sun-like star

4) When the star uses up all the hydrogen in its core, the hydrogen in the shell (a larger region surrounding the core) ignites.

“life cycle” of a Sun-like star

5) The energy released by the burning of the hydrogen shell makes the star brighter and expands it, making its surface larger, cooler, and redder so it becomes a so-called red giant star

Figure 14-6

A Hertzsprung-Russell diagram plots a star’s temperature versus its energy output.Stars in the hydrogen-burning stage, including the Sun, lie along the main sequence, while red giants and white dwarfs represent subsequent stages of stellar life.

“life cycle” of a Sun-like star

6) Stellar winds blowing off the star gradually expel its outer layers, which form a planetary nebula around the remaining hot stellar core

“life cycle” of a Sun-like star

7) The old star dissipates into space, leaving just the hot little core.

This white dwarf star, cools and fades forever.

StarStuff

• Elements heavier than H and He are made in star cores

• All atoms on the earth heavier than helium were once part of another star

Figure 14-6

A Hertzsprung-Russell diagram plots a star’s temperature versus its energy output.Stars in the hydrogen-burning stage, including the Sun, lie along the main sequence, while red giants and white dwarfs represent subsequent stages of stellar life.

Life of Stars

• The life span of a star depends on the rate at which it burns its fuel

• High mass stars burn fast, bright, and are shorter lived

• Low mass stars burn slow, dim, and live longer

Death of Stars

• star expands to become a red giant

• Eventually cools

and contracts to

become a white dwarf

What happens to the stellar remnant?

• may continue to radiate energy as it cools, but ultimately, it fades to a cold black lump of matter (black dwarf)

• white dwarf might (by its own gravity) pull H from its companion star and ignites to embroil the white dwarf in a thermonuclear holocaust that we see as a nova

Figure 14-7

The life cycle of the Sun on a Hertzsprung-Russell diagram. The Sun started hydrogen burning in its core more than 4.5 billion years ago on the main sequence (at point 1), and it will remain near that point on the diagram for several billion years more. As the hydrogen in the core is consumed, however, a short period of helium burning (point 2) will move the Sun’s position on the diagram rapidly upward toward the red giant stage (point 3). Once the helium is consumed, the nuclear fusion reactions will cease and gravitational collapse will cause the Sun to heat up (point 4). Eventually the Sun will cool to a white dwarf (point 5).

White Dwarf Supergiants

Figure 14-6

A Hertzsprung-Russell diagram plots a star’s temperature versus its energy output.Stars in the hydrogen-burning stage, including the Sun, lie along the main sequence, while red giants and white dwarfs represent subsequent stages of stellar life.

nova

• Possible fate for white dwarf

• In binary stars,• white dwarf may pull

H from companion star and re-ignite.

Supernova

• More massive stars • Nuclear fusion forms

higher elements • Fe and higher at #s• Core eventually

collapses and rebounds in an explosion

Black Holes

• Collapsed stars that attract their own light

• By far, the most dense stellar remnant

• So much mass packed in a small space that not even light can get out!

For review…

• try discussion questions 1-7, 9-11, 14, 15 on page 296

• Please consider the Whitman poem given in Investigations # 5, and

• by all means, when the night sky is clear, go out and stare at the stars! ;o)

Cosmology

SCI 2201

Ch 15

Great Idea:

The universe began billions of years ago

in the big bang,

and it has been expanding ever since.

Milky Way Galaxy

Milky Way Galaxy

Cosmology

• 1924, Edwin Hubble established the fact that the Milky Way is just one of countless galaxies

• cosmology – study of the structure and history of the entire universe

Kinds of galaxies

• –2 major types, see top p325

• Spiral

• Elliptical

• Dwarf

“redshift”

• Hubble was first to notice that colors emitted by different elements seemed to be shifted toward the red (long-wavelength) end of the spectrum.

• --pitch drops as moving source of sound travels away from you

• --light shifts to red as it moves away from you

Hubble’s observation meant that distant galaxies are

moving away from Earth!

Hubble’s Law

• The farther away a galaxy is, the faster it recedes.

Hubble’s Law tell us:

• the universe is expanding.

• The whole cosmos is blowing up like a balloon

• The universe began at a specific point in the past, and it has been expanding ever since.

Evidence for Big Bang

• explained pp 306-308

• Universal Expansion

• Cosmic Microwave Background

• Abundance of Light Elements