Stars can be grouped into constellations. VOCABULARY Stars and Their Characteristics Stars and Galaxies Hydrogen and helium are the two most abundant elements

Stars can be grouped into constellations.

VOCABULARYStars and Their Characteristics

Stars and Galaxies

Hydrogen and helium are the two most

abundant elements in stars.

HERCULES

BOÖTES

COMA BERENICES

CYGNUS

CEPHUS LACERTA

CASSIOPEIA

CANES VENATICI

LEO

LEO MINOR

URSA MAJOR

URSA MINOR

LYNX

DRACO

PERSEUS

PEGASUS

PISCES

LYRA

CAMELOPARDALIS

constellation

apparent magnitude

astronomical unit

light-year

parsec

luminosity

absolute magnitude

Stars and Their Characteristics


Stars and Galaxies

Stars differ in mass, size, and surface temperature.

Surface temperature affects the color of stars.

Apparent magnitude, luminosity, and absolute

magnitude are used to describe the brightness of

stars.

There are stars of different brightness in the constellation Orion, including two of the brightest stars as viewed from Earth—Betelgeuse and Rigel.

Betelgeuse

Rigel


Distances in space are measured in astronomical

units, light-years, and parsecs.

Stars that show variation in brightness are known

as variable stars.

Light-Year The distance that light travels in one year, about 9.5 trillion kilometers.

Parsec A unit of measurement used to describe distances between celestial objects, equal to 3.258 light-years.

Spectral Types

Apparent Magnitude

Some stars appear very bright but are actually fainter stars that lie closer to us. Similarly, we can see stars that appear to be faint, but are intrinsically very bright ones lying far away from Earth.

Apparent Magnitude The measure of how bright a star appears to be to an observer on Earth.

Apparent Magnitude


Luminosity: The brightness of a star or the power radiated by the star.

The luminosity is a quantity that depends on the star itself, not on how far away it is. For this reason a star's luminosity tells you about the internal physics of the star and is a more important quantity than the apparent brightness.

What does the luminosity of a star depend on?

Temperature (proportional to T4)

Size (proportional to R2)

Full blown formula? L=4R2T4

A star can be luminous because it is hot or it is large (or both!).

The luminosity of an object = the amount of energy every square meter produces multiplied by its surface area.

Caution!

Do not confuse the size of an object with the mass of an object.

Just because an object is large in dimension does not necessarily mean it is also large in mass.

For example, you can have a forty foot tall by three foot across marshmallow that looks “large,” but that does not mass as much as that of a “small” football sized hunk of lead.

Absolute Magnitude

Absolute Magnitude: The measure of how bright a star would be if it were located 10 parsecs from Earth.

On the left-hand map of Canis Major, dot sizes indicate stars' apparent magnitudes; the dots match the brightnesses of the stars as we see them.

The right-hand version indicates the same stars' absolute magnitudes — how bright they would appear if they were all placed at the same distance (32.6 light-years) from Earth. Absolute magnitude is a measure of true stellar luminosity.

Inverse Square LawAs the light from a star goes into space it fills a

larger and larger spheres.The area of a sphere is given by its radius: A = 4

d2

d is the radius of the sphere

The amount of light we The amount of light we receive from a star receive from a star decreases with the decreases with the square of our distance square of our distance from the star:from the star:

Amount of light = LAmount of light = L00 / d / d22

Flux=“amount of light”

Measuring the Distance to Stars

Measuring distances is difficult.

The best method for measuring distances The best method for measuring distances of nearby stars is called of nearby stars is called parallax.parallax.

It relies on observing a star from It relies on observing a star from two two different places.different places.

Measuring the Parallax Angle:The parallax angle p is illustrated in the following figure.


Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the apparent shift of an object against a background caused by a change in observer position.


The Distance to the StarsWe obtain a different perspective

on a star by observing it at different times of the year.

In 6 months the Earth has moved 2 AU away.

(2AU = 300 million km)

The parallax method lets us measure the distance to stars about 1000 light years away.

Measuring Distances: Parallax

The larger the star’s distance, d, the smaller its parallax p.

So distance and parallax are inversely related. d = 1 / p

Measuring Distances: Parallax

Most stars have a parallax angle, p, which is very small.

The angle of parallax, p, is usually measured in arc seconds

60 arc seconds = 1 arc minute

60 arc minutes = 1 degree.

Distances to stars are measured in either: light years, or parsecs.

1 parsec = 3.2 light years

If a star’s parallax is 1 arc second, then its distance is 1 parsec.

(parsec = PARallax of one arcSEC)

Parallax Examples

If a star’s parallax is 1 arc second its distance is 1 parsec

Question: If a star has a parallax of 0.1 arc seconds what is its distance in parsecs?

Answer: d = 1 / p

d = 1/ (0.1) = 10 parsecs = 3.2 light years

Constellations: A group of stars that appear to form a pattern in the sky.

Constellations are easily recognizable patterns that help people orient themselves using the night sky. There are 88 “official” constellations.

Hundreds of the brightest stars, those visible with the unaided eye, were given names in ancient times.

Today stars are named by their coordinates on the celestial sphere. This is an imaginary sphere surrounding Earth.

Constellations

All stars and objects in space, can be mapped relative to the poles and equator of the celestial sphere. Their position north or south of the celestial equator — essentially their latitude — is called “declination.” Their position east or west essentially is their longitude, or right ascension, measured in hours, minutes, and seconds.

Constellations

The stars are distant objects. Their distances vary, but they are all very far away. Excluding our Sun, the nearest star, Proxima Centauri, is more than 4 light years away. As Earth spins, the stars appear to move across our night sky from east to west, for the same reason that our Sun appears to “rise” in the east and “set” in the west.

Constellations

If observed through the year, the constellations shift gradually to the west. This is caused by Earth’s orbit around our Sun. In the summer, viewers are looking in a different direction in space at night than they are during the winter.

Constellations

Stars close to the celestial poles, the imaginary points where Earth’s north and south axes point in space, have a very small circle of spin. Polaris, Earth’s north “pole star,” will appear to move very little in the night sky. The farther from Polaris, the wider the circle the stars trace.

Constellations

Stars that make a full circle around a celestial pole, like those in the Big and Little Dippers in the northern hemisphere, are called “circumpolar stars.” They stay in the night sky and do not set. At the equator, there are no circumpolar stars because the celestial poles are located at the horizon. All stars observed at the equator rise in the east and set in the west.

Constellations

Constellations

HERCULES

BOÖTES

COMA BERENICES

CYGNUS

CEPHUS LACERTA

CASSIOPEIA

CANES VENATICI

LEO

LEO MINOR

URSA MAJOR

URSA MINOR

LYNX

DRACO

PERSEUS

PEGASUS

PISCES

LYRA

CAMELOPARDALIS

The Hertzsprung-Russell diagram plots a star’s

luminosity against its surface temperature. The

diagram’s groups

of stars

represent life-

cycle stages of

stars. Most stars

are main-

sequence stars.

VOCABULARYLife Cycles of Stars

Stars and Galaxies

Hertzsprung-Russell diagram

Temperature

Lu

min

os

i ty

Hottest Coolest

Hig

he

st

Lo

we

st

Blue Supergiants Red Supergiants

Red Giants

Red DwarfsWhite

Dwarfs

<Main Sequence>

main sequence

giant star

supergiants

white dwarf

nebula

planetary nebula

supernova

neutron star

pulsar

black hole

Hertzsprung-Russell Diagram:

– Ejmar Hertzsprung (1873-1967) – Copenhagen – Began his career as a Chemical Engineer. While working and independently at the same time…

– Henry Norris Russell (1877-1957) – Princeton – Student then professor.

– A graph that separates the effects of temperature and surface area on stellar luminosities.

– The HR Diagram is much like the same thing as producing a graph of people’s height vs. weight.

A star’s fate depends on its mass.

Life Cycles of Stars

Stars and Galaxies

A star with a mass similar to the sun’s will become

a white dwarf.

A star with a mass eight or more times greater

than the sun’s will either become a black hole

or a neutron star.

Life Cycles of Stars

Stars and Galaxies

Properties of stars

Color and temperature • Hot star

• Temperature above 30,000 K

• Emits short-wavelength light

• Appears blue

• Cool star • Temperature less than 3000 K

• Emits longer-wavelength light

• Appears red

Properties of stars

Color and temperature • Between 5000 and 6000 K

• Stars appear yellow

• e.g., Sun

Binary stars and stellar mass • Binary stars

• Two stars orbiting one another

• Stars are held together by mutual gravitation• Both orbit around a common center of mass


Shows the relation between stellar • Brightness (absolute magnitude) and• Temperature

Diagram is made by plotting (graphing) each star's • Luminosity (brightness) and• Temperature


Parts of an H-R diagram • Main-sequence stars

• 90% of all stars

• Band through the center of the H-R diagram

• Sun is in the main-sequence

• Giants (or red giants) • Very luminous

• Large

• Upper-right on the H-R diagram


Parts of an H-R diagram • Giants (or red giants)

• Very large giants are called supergiants• Only a few percent of all stars

• White dwarfs• Fainter than main-sequence stars• Small (approximate the size of Earth)• Lower-central area on the H-R diagram• Not all are white in color• Perhaps 10% of all stars


Birth of a Star: Nebula

Stars are born in a glowing cloud of interstellar gas and dust (mostly hydrogen), called a nebula.

Gravity causes every atom and every bit of dust to pull on every other one and all move to the center, causing the protostar to collapse.

Because the atoms move faster and faster as they fall toward the center, friction is created as they rub together and the temperature rises.

Heat causes the protostar to glow in with its own light, giving off even more light than our Sun even though it is not nearly as hot.

When a temperature of about 27,000,000°F is reached, nuclear fusion begins. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the star.

The protostar is now a stable main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die.



Black Widow Nebula


Crab Nebula


Main Sequence Stars

Main Sequence: A star that is at the point in its life cycle in which it is actively fusing hydrogen nuclei into helium nuclei;

Our sun is a main sequence star.

A Giant Star: is large star with great luminosity and a diameter 10 to 100 times greater than that of the sun.

Giant Stars

A giant star is one of two kinds very large stars the other being a Red giant or Supergiant

Red giants are stars of 1000 times the volume of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.

Supergiants: are the most luminous, most massive stars, with diameters greater than 100 times the diameter of the sun.

Supergiants

The best known example is Rigel, the brightest star in the constellation of Orion. It has a mass of around 20 times that of the Sun and gives out more light than 60,000 suns added together.

A white dwarf is what stars like our Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula.

White Dwarf : The remnant of a giant star that has lost its outer atmosphere; the glowing stellar core.

White Dwarf:

Sirius–B, this whitedwarf is very hot due to high density and rapid spin.

A neutron star is formed from the collapsed remnant a Type II,

Type Ib, or Type Ic supernova.

Neutron star: The superdense remains of a

massive star that collapsed with enough force to push all of its electrons into the nuclei they orbit, resulting in a mass of neutrons.

•Pulsar – general term for neutron stars that emit directed pulses of radiation towards us at regular intervals due to their strong magnetic fields.

Supernovae are the main source of all the elements heavier than oxygen, and they are the only source of many important elements.

Supernova: The brilliant burst of light that follows the collapse of the iron core of a massive star.

X-ray image of the remnant of Kepler's Supernova

Supernova Remanent

Black hole: The final life stage of an extremely massive star, with a gravitational field so intense that not even light can escape.

Black holes are areas in space where there is a huge amount of mass in a very small space. The gravity of this mass is so great that everything in the area is pulled toward the mass. Even light, with its tiny mass, is pulled into the center of the hole. No object can escape the gravitational pull of a black hole.

We can't see a black hole because no light escapes the event. Astronomers use other ways to look for black holes. Since they have large masses and gravities, they affect the surrounding stars and systems. They have found evidence of black holes in the dark centers of galaxies and systems that emit large amounts of x-rays.


How does a black hole form?

A black hole forms when any object reaches a certain critical density, and its gravity causes it to collapse to an almost infinitely small pinpoint. Stellar-mass black holes form when a massive star can no longer produce energy in its core. With the radiation from its nuclear reactions to keep the star "puffed up," gravity causes the core to collapse. The star's outer layers may blast away into space, or they may fall into the black hole to make it heavier.

Supermassive black holes containing millions to billions of times the mass of the sun are believed to exist in the center of most galaxies, including our own Milky Way.

Intermediate-mass black holes, whose size is measured in thousands of solar masses, may exist. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources.


VOCABULARY

Galaxies and the Universe

Galaxies contain

millions or billions

of stars. There are

three major types

of galaxies: spiral,

elliptical, and

irregular.

galaxy

quasar

Normal galaxies emit as much radiation as that

given off by their stars. Active galaxies emit much

more radiation than that given off by their stars,

possibly due to supermassive black holes at their

center.

Quasar: A very distant, extremely luminous

celestial object that scientists consider to be

a type of active galactic nuclei.


Galaxies are defined as large groupings of stars, dust, and gas held together by gravity. They vary greatly in size and shape. Most of the objects we know of in space are contained within galaxies. They contain stars, planets, moons, comets, asteroids, nebulae, dust, neutron stars, and black holes. Many probably even contain large amounts of unseen dark matter. Since most of the space between galaxies is thought to be empty, a galaxy is essentially an oasis in space.


Spiral Galaxy - Spiral galaxies are characterized by a distinct flattened spiral disk with a bright center called the nucleus. Our own Milky Way is a spiral galaxy. Spiral galaxies are represented by the letter S and are divided into subgroups.

Galaxies and the Universe: Types of Galaxies


Barred Spiral Galaxy - A barred spiral galaxy is very similar to a spiral with one important difference. The arms spiral out from a straight bar of stars instead of from the center. About one third of all spiral galaxies are barred spiral in shape.



NGC 1365 is one of the most prominent barred galaxies in the sky. It is a supergiant galaxy with a diameter of about 200 000 light years.

Elliptical Galaxy - Elliptical galaxies vary in shape from completely round to extremely elongated ovals. Unlike spiral galaxies, they have no bright nucleus at their center. Elliptical galaxies are represented by the letter E and are divided into seven subgroups according to their shape.



NASA's Chandra X-ray Observatory shows hot gas in nine different elliptical galaxies.

Irregular Galaxy - A fourth type of galaxy is known as the irregular galaxy. These galaxies have no discernable shape or structure. Irregular galaxies are divided into two classes, Im and IO. Im class galaxies are the most common and show just a hint of structure. Sometimes the faint remnants of spiral arms can be seen. IO class galaxies are completely chaotic in form.


Irregular galaxy, as observed by the Hubble Space Telescope.



Paired Galaxies

Life Cycle of a Star

Colliding Galaxies

Galaxy Cluster

Galactic Research

Edwin Hubble was the first person to figure out how to tell the distance of a galaxy. He used a type of pulsating star known as a Cepheid variable as a kind of galactic yardstick.

Hubble noticed a correlation between the period required to complete one pulsation brightness and the amount of energy the star gives off. This was the first major breakthrough in galactic research. Hubble also discovered that there was a correlation between the red shift of a galaxy and its distance. This is known today as the Hubble constant.

Galactic Research

Today astronomers are able to measure the speed and distance of a galaxy by measuring the amount of redshift in its spectrum. We know that all galaxies are moving away from each other. The farther a galaxy is from us, the faster it is moving.

Galactic Research

Documents

Stars can be grouped into constellations. VOCABULARY Stars and Their Characteristics Stars and Galaxies Hydrogen and helium are the two most abundant elements