CHAPTER 28 STARS AND GALAXIES

28.1 A CLOSER LOOK AT LIGHT

Light is a form of electromagnetic radiation, which is energy that travels in waves.

Waves of energy travel at 300,000 km/sec (speed of light

Ex: radio waves and x-rays

Electromagnetic waves do not need a medium to travel, they travel through space

Electromagnetic waves emitted by an object indicated what elements are present and its motion

Electromagnetic radiation waves are arranged into a continuum call the electromagnetic spectrum.

Wide range of wavelengths

Long wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other end

Wavelength measured crest to crest/trough to trough

Frequency the number of that crests of the same wavelength that pass a point in one second.

Scientists study the visible light portion of the electromagnetic spectrum.

Spectra of a stars allow for astronomers to learn about the star’s elements and motion.

Spectra studied using a spectroscope

Three types of visible spectra Continuous spectrum: unbroken band of colors, emitting all

colors of visible light glowing solids, such as a light bulb filament glowing liquids, molten iron hot, compressed gases inside a star

Emission spectrum: unevenly spaced lines of different colors, emitting light of only some wavelengths

glowing thin gases every element has its own unique emission spectrum, element’s finger print

Absorption spectrum: dark lines that cross a continuous spectrum.

forms when light from a glowing object pass through a cooler gas which absorbs some wavelengths

can determine what is present in the cooler gas by comparing emission and absorption spectrum

Doppler Effect is the apparent change in the wavelength of radiation or sound due to relative motion between the object and the receiver.

Doppler effect applies to light as well as sound. Shift of the emission spectra can indicate if the object

is moving towards or away from Earth

Shift towards the red end of the visible light spectrum, object moving away Earth – Redshift

Shift towards the blue end of the visible light spectrum, object moving towards Earth - Blueshift

Doppler effect determined that the universe is expanding

TELESCOPES Optical Telescopes – gather far more light than

an unaided eye and magnify imagers

Reflecting

Uses one lens at back to gather and focus light

Image reflected on to a small mirror and then the eye piece

Refracting

Uses two lenses

Lens at the front gathers light

Eyepiece magnifies image

Radio Telescopes Technical term for really big satellite dishes

Use to detect energy waves at frequencies lower than visible light

Other Detects energy waves at frequencies higher than

visible light

Usually satellites in space Gamma ray

Background radiation

X-ray

Hubble (infrared)

28.2 Stars and Their Characteristics Observation of stars has been going on for over

5000 years

The grouping of stars are called constellations

Constellations human grouping of stars

only appear together as viewed from Earth from a different angle they do not look like the constellation

Grouped together due to looking like they all have the same brightness (apparent magnitude)

Constellations (continued) Constellations will change

shape over thousands of years due to the universe expanding

Some constellations have been around for thousands of years other were recently conceived

Move across the sky from east to west (though Earth rotates west to east)

Big Dipper and Little Dipper two of the best know constellations point to the North Star

North Star Current north star is called

Polaris

Sits directly over the North pole

Does not move to the naked eye

Very powerful tool for navigation

Due to Precession, Polaris will not always be the “North Star”

Circumpolar Constellations – constellations around the north star

Create star trails evidence for Earth’s rotation

North star appears fixed in the sky as Earth rotates

star trails from circumpolar constellations rotate counterclockwise around the north star

Circumpolar constellations seem to never set from some northern hemisphere latitude

The Constellations that dominate the night sky change from month to month. This is the result of the Earth’s change in position as it orbits the Sun.

Plain of the Ecliptic (path of Sun, Moon and Planets across the sky)

Curve in Plain of the Ecliptic is due to change in tilt of the Earth over the course of a year.

Distances to stars and other objects in space

Unit of measurements on Earth are not sufficient for space measurements

Astronomical Unit (AU) is used for the distance from Earth to the Sun (150 million kilometers)

Light year is the distance light travels in one year (9.5 trillion kilometers) It is a distance measurement

Example - 4.2 light-years means that the light we see has been traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km)

Parallax change in an object’s direction due to a change in the observer’s position

Parsec short for “parallax second” equal to 3.258 light-years.

Parallax

The further the object from the viewer, the less the parallax shift.

Stars

Elements

Hydrogen ~69%

Helium ~29%

Heavier elements ~2%

No two stars have the same proportions of elements

light that radiates is dependent on composition and temperature, this differs in every star

Star spectrum is its fingerprint

Mass, Size and Temperature Stars vary greatly in masses, size and temperature

Cannot observe directly so we are estimating what the mass might be Gravitational effect on bodies around the star help with

estimating its mass

Star mass is expressed as multiples of the mass of our Sun (which has a stellar mass of 1)

Size varies more than mass Smallest stars are smaller than Earth

Largest have diameters more than 2000 times that of our Sun

Stars differ even more in density Betelguese is about one ten-millionth of our Sun

One star is so dense that one teaspoon would weight more than a ton on Earth

Star size comparison

Temperature of stars vary

Stars have Different colors which indicate different temperatures

Cool stars are redder in color Ex. Betelguese with a surface temperature of 3000oC

Mid-temperatures are yellower in color Ex. The Sun with a surface temperature of 5500oC

Hot stars are blue in color Ex. Sirus

Astronomers group stars by temperature and color into spectral classes.

System is called the Harvard Spectral Classification Scheme

Actual brightness of a star is Luminosity Dependent on size and temperature of the star

Distance from Earth not a factor

If two stars are the same size the hotter star would be more luminous

Apparent magnitude is how bright a star appears to an observer on Earth Does not factor in distance

Absolute magnitude is the measure of how bright the star would be if all stars were the same distance from Earth (10 parsec)

Variable stars

These stars show a variation in brightness over cycles

Can last days to years

Some change brightness as they expand and contract

One important class is called Cepheid variables

Yellow supergiant stars with a cycle of brightness ranging from 1- 50 days.

Most have cycle of 5 day.

Nonpulsating star may change in brightness due to the fact that is more than one star.

28.3 LIFE CYCLE OF STARS

Stage 1

Stage 3 Stage 2

•Main sequence stage, last

the longest

•Star stays until all H2 fuel

is used up (converted to He)

•He core shrinks and

Contracts to produce

additional heat.

•H2 fusion starts in the outer

Layers causing the star to

swells– moves to next stage.

Main Sequence Star (stars like our Sun) less than 8 solar masses

•Nebula spins & flattens

To form a protostar

•Protostar shrinks due to

gravitional attraction.

•As temperature increases

and pressure increases,

light and heat are emitted

•Energy is produced from

Nuclear fusion of H2 into

He begins – moves to next

stage

•Core temperature increases

to a point that He can fuse into

heavier elements (O2 and C)

• H2 into He continues in the

outer layers.

•When Hydrogen is exhausted

the outer layers blow away

leaving only the carbon-oxygen

core – White Dwarf.

•Expelled layer absorbs the

white dwarfs ultraviolet emission

causing a halo affect called a

Planetary Nebula

Massive Star 8 solar masses or more

•Same process however due

to the more massive size it

swells to an even larger size

than the main sequence star

does

•Same process however

much more dust and gas

present

•Fusion in core continues until iron

forms

•Iron does not release energy so it

absorbs it, thus leading to a quick

collapse resulting in an explosion –

Supernova.

•Massive star that is 8-15 solar

masses, creates a neutron star

following the supernova

•Massive star that is more than

15 Solar masses, creates a

black hole following the supernova.

HR Diagram

HR diagram gives you a picture of a star’s life. There are thousands of variations of the HR

diagram

All plot luminosity against temperature

Some include spectral class and absolute magnitude. Spectral class is based on the temperature of the star

Luminosity is the actual brightness of a star, where as absolute magnitude is the brightness if all stars were the same distance from Earth.

HR Diagram Groups

Stars fall into one of several distinct groups on the HR diagram.

90% of stars fall in a band called the main sequence, which runs from the upper left corner to the bottom right corner. Stars in this band are called main sequence stars.

Giant stars grouping is found in the upper right corner just above the main sequence. Larger stars are more luminous and have diameters from 10

to 100 times greater than the sun

HR Diagram Groups

Supergiants grouping is found in the upper right corner just above the giant stars. Diameters that are more than 100 times that of the sun

Relatively cool stars but their size gives them their luminosity.

White Dwarfs grouping is found in the lower left corner, these stars are near the end of their lives. Were once red giants

The red giants outer lost their outer atmosphere, leaving only a glowing stellar core.

Black holes and Neutron stars do not appear on the HR diagram because their luminosity is not great enough.

28. 4 Galaxies and the Universe

The universe is everything that exists.

The observable universe is everything we can observe.

Astronomers are not sure how old the universe is since the light from the beginning has not yet reach us. The estimate is between 10-20 billion years.

Galaxies and the Universe

Galaxies are hard to separate from stars without telescopes.

Hazy patches of light which when viewed through a telescope reveals thousands to billions of stars.

Astronomers estimate there are 50- 100 billion galaxies in the observable universe.

No two galaxies are the same, however, they are classified based on shape.

Galaxies and the Universe

There are three classifications

Spiral (ex. Milky Way)

Elliptical are near spherical to lens-shape

Irregular (ex. The two Magellanic Clouds)

Spiral Irregular Elliptical

Big Bang Model – explains history of the universe from a tiny fraction of a second after it came into being up to the present Best explanation for how the universe came to being Developed due to observations of stars, galaxies and

other objects with telescopes and experimenting with matter on Earth

Approximately 10-20 billion years ago all matter in the universe existed in an incredibly hot and dense state, from which it expanded and cooled slowly condensing into stars and galaxies.

It is expanding at a very slow rate and is still expanding today

Origin of the Universe

Evidence for the Big Bang Model: The universe’s apparent expansion, distance between

galaxies and groups of galaxies seems to be increasing with time.

Edwin Hubble’s discovery of the redshift in the spectra of galaxies supports the expansion of the universe

Discovery of radiation called cosmic background radiation apparently left over from the universe’s beginning.

Continues to be tested and examined to seek further evidence to support it

Some astronomers are considering alternative ways the universe has reached its present state.

Origin of the Universe

It is impossible to know for certain how the universe began

as long as the Big Bang Theory (model) survives crucial tests, it remains the best explanation for the origin of the universe

If it were to ever fail a test, then astronomers will have to look for a new theory/model

Origin of the Universe