Lesson 1-1 The electromagnetic spectrum is a continuous range of wavelengths. Looking at the Night...

The electromagnetic spectrum is a continuous range of wavelengths.

Looking at the Night Sky (cont.)

Different parts of the electromagnetic spectrum have different wavelengths and different energies. You can see only a small part of the energy in these wavelengths.

An astronomical unit is the average distance between Earth and the Sun, about 150 million km.

• A light-year is the distance light travels in 1 year.

• One light-year is about 10 trillion km.

Measuring Distances (cont.)

The apparent magnitude of an object is a measure of how bright it appears from Earth.

Measuring Brightness

• Luminosity is the true brightness of an object.

• The luminosity of a star, measured on an absolute magnitude scale, depends on the star’s temperature and size, not its distance from Earth.

Measuring Brightness (cont.)

• nuclear fusion

• star

• radiative zone

• convection zone

The Sun and Other Stars

• photosphere

• chromosphere

• corona

• Hertzsprung-Russell diagram

How Stars Shine• A star is a large ball of gas held together

by gravity with a core so hot that nuclear fusion occurs.

• Nuclear fusion occurs when the nuclei of several atoms combine into one larger nucleus.

How Stars Shine (cont.)

• Nuclear fusion releases a large amount of energy.

• A star shines because when energy leaves a star’s core, it travels throughout the star and radiates into space.

Spectra of the Sun and other stars provide information about stellar composition.

Composition and Structure of Stars

stellar

Science Use anything related to stars

Common Use outstanding, exemplary

• There are three interior layers of a typical star.

• When first formed, all stars fuse hydrogen into helium in their cores.

Composition and Structure of Stars (cont.)

• The radiative zone is a shell of cooler hydrogen around a star’s core.

• In the convection zone, hot gases move toward the surface as cooler gases move down into the interior.

Composition and Structure of Stars (cont.)

Beyond the convection zone are the three layers of a star’s atmosphere—the photosphere, the chromosphere, and the corona.

Composition and Structure of Stars (cont.)

The photosphere is the apparent surface of a star, where light energy radiates into space.

Composition and Structure of Stars (cont.)

• The chromosphere is the orange-red layer above the photosphere.

• The corona is the wide, outermost layer of a star’s atmosphere.

Composition and Structure of Stars (cont.)

Changing Features of the Sun: Sunspots

• Cooler regions of magnetic activity

• Seem to move as the Sun rotates

• Number varies on an 11-year cycle

Digital Vision/PunchStock

Changing Features of the Sun: Coronal Mass Ejections (CMEs)

• Huge gas bubbles ejected from the corona

• Larger than flares

• May reach Earth

• Can cause radio blackouts

NASA

Changing Features of the Sun: Prominences and Flares• Prominences—

clouds and jets of gases forming loops into the corona

• Flares—sudden increases in brightness, often near sunspots or prominences

SOHO Consortium, ESA, NASA

Changing Features of the Sun: The Solar Wind• Caused by

charged particles streaming away from the Sun

• Extends to the edge of the solar system

• Causes auroras CORBIS

• Most stars exist in star systems bound by gravity.

• Many stars exist in large groupings called clusters.

• Stars in a cluster all formed at about the same time and are the same distance from Earth.

Groups of Stars

• Scientists classify stars according to their spectra.

• Though there are exceptions, color in most stars is related to mass.

Classifying Stars

Blue-white stars tend to have the most mass, followed by white stars, yellow stars, orange stars, and red stars.

The Hertzsprung-Russell diagram is a graph that plots luminosity against temperature of stars.

The y-axis of the H-R diagram displays increasing luminosity and the x-axis displays decreasing temperature.

Most stars exist along the main sequence.

The mass of a main-sequence star determines both its temperature and its luminosity

• nebula

• white dwarf

• supernova

• neutron star

• black hole

Evolution of Stars

• Stars form deep inside clouds of gas and dust.

• A cloud of gas and dust is a nebula.

Life Cycle of a Star

nebula

from Latin nebula, means “mist” or “little cloud”

Star-forming nebulae are cold, dense, and dark.

Life Cycle of a Star (cont.)

Photo courtesy of NASA/Corbis

• Gravity causes the densest parts to collapse, forming regions called protostars.

• Over many thousands of years, the energy produced by protostars heats the gas and dust surrounding them.

Life Cycle of a Star (cont.)

• A star becomes a main-sequence star as soon as it begins to fuse hydrogen into helium.

• Low-mass stars stay on the main sequence for billions of years, and high-mass stars are there for only a few million years.

Life Cycle of a Star (cont.)

• When a star’s hydrogen supply is nearly gone, the star leaves the main sequence and begins the next stage of its life cycle.

• All stars form in the same way, but stars die in different ways, depending on their masses.

• Massive stars eventually become red supergiants.

Life Cycle of a Star (cont.)

• After helium in the cores of lower-mass stars is gone, the stars cast off their gases exposing their cores.

• The core eventually becomes a white dwarf, a hot, dense, slowly cooling sphere of carbon.

• This is what is expected to happen to the Sun.

End of a Star

The Sun will remain on the main sequence for 5 billion more years.

When the Sun becomes a red giant for the second time, it will probably absorb Earth and push Mars and Jupiter outward.

When the Sun becomes a white dwarf, the solar system will be a cold, dark place.

• Very massive stars can explode in a supernova, which destroys the star.

• Iron in the core does not fuse and the core collapses quickly under the force of gravity.

• The normal space within atoms is eliminated, leaving a dense core of neutrons, or a neutron star.

End of a Star (cont.)

• For the most massive stars, atomic forces holding neutrons together are not strong enough to overcome so much mass in such a small volume. Gravity is too strong, and the matter crushes into a black hole.

• A black hole is an object whose gravity is so great that no light can escape.

End of a Star (cont.)