# Stars & Their Characteristics

## Presentation on theme: "Stars & Their Characteristics"— Presentation transcript:

Stars & Their Characteristics

Constellations A group of stars that appear to form a pattern in the sky. Only appear together from Earth, actually at dif. Distances. Far distance of stars make any movement not apparent in a constellation for thousands of years. Asterism- small grouping of stars. Ex. Big Dipper part of constellation Ursa Major (Big Bear). Can find Polaris (North Star) using Big Dipper.

Star Movement/Sky Appearance
Movement of stars in sky caused by Earth’s rotation & revolution. Earth turns West to East so sky appears to move from East to West to us on Earth, that’s why the sun, moon, & stars rise in the East & set in the West. Sky directly above poles seems stationary as Earth turns on it’s axis, so Polaris appears fixed in the sky while all other stars seem to move counter-clockwise around it. These stars are Circumpolar Stars.

Earth’s Movement & Constellations
Constellation positions in the sky change with the seasons. Ex. Big Dipper: Fall-near North horizon, Spring- overhead. Some constellations can only be seen during certain seasons. Ex. Lyra in Summer & Orion in Winter. These changes are due to Earth’s position as it orbits the Sun. Do demo with Large Ball (sun), small ball (earth), & paper labeled Lyra & Orion. Demonstrate orbit & eclipsing of constellations. Ask: Where are circumpolar stars located? (above earth)

Apparent Magnitude A measure of how bright a star appears to be to an observer on Earth. Magnitude is measured on a scale from 1 (brightest)- 6 (faintest). The lower a stars magnitude #, the brighter the star.

Distances to stars 3 units of measurements
Because any measurement we use on earth is too small, measuring in space requires larger units of measurements.

1. AU (Astronomical Unit)
Distance from the Sun to the Earth. Equal to 1.5 x 108 Km or 150 million Km.

2. Light-Year Measures the distance that a ray of light travels in one year. In one year, light travels 9.5 x 1012 Km or 9.5 trillion Km. Ex: Proxima Centauri is 4.2 light years from Earth.

3. Parsec (Parallax Second)
Measures distances between celestial objects (nearest stars). One parsec = light-years or x 1013 Km. Parallax- change in objects direction due to change in observers position. Occurs because Earth orbits the sun. Using parsec measurement accounts for the shift by knowing the angel between the 2 points.

Elements in Stars Each star is different, but they are mostly composed of H, He & some heavier elements. A stars spectrum is used to determine it’s composition. Each star has a unique spectrum.

Mass, Size & Temp. of Stars
Vary more in size than in mass. Mass= total amount of material in a body. Determined by gravitational influence on other bodies. Stellar (star) masses are expressed as multiples of the sun or One Solar Mass.

Temperature & Color of Stars
Range of color a star emits depends on it’s surface temp. To us on earth, stars look mostly white with a bit of color. Cooler stars= reddish, Hotter stars= Bluish-white. Ex: Iron when heated, 1st turns red, changes to orange, to yellow to white. Very Hot objects glow Bluish. The color/temp. of a star helps astronomers determine the elements present. Recall, every element when heated emits it’s own bright light or emission spectrum.

Luminosity & Absolute Magnitude
Actual brightness of a star. Depends on size & temp. Ex: If 2 stars had the same surface temp, but 1 was bigger, the bigger is more luminous. If 2 stars were same size, but dif. temps, the hotter is more luminous True measure of how bright a star is. What’s the difference between apparent & absolute magnitude? Demo: Bed sheet & flashlights at dif. & same distances then ask: 1. when did you compare the apparent magnitudes of light? (when flashlights at dif distances from sheet) 2. when did you compare the absolute magnitudes of light? (when flashlights at same distances from sheet)

Life Cycles of Stars Stars are born from great clouds of gas & dust. They mature, grow old, & die. When they die, they produce new clouds of dust, from which new stars may arise, along w/planets to orbit them. The more massive a star, the shorter its life will be. Stars are born from great clouds of gas & dust. They mature, grow old, & die. When they die they produce new clouds of dust, from which new stars may arise, along w/planets to orbit them. The more massive a star, the shorter its life will be.

What is the H-R diagram? Hertzsprung-Russell diagram – a diagram that plots the stars surface temperature against their absolute magnitude.

Groups of the H-R Diagram:
Groups represent stages in the life cycles of the stars. 90% of stars fall in the Main Sequence band & range in size & temp. these are the stars that are fusing H to He. Giant Stars are above main sequence & are more luminous & larger. Supergiants are even more luminous & large. White Dwarfs are near the end of their lives, were once red giants but lost atmosphere & now only glowing stellar core. Ask: 1. where are the bright, cool stars? 2. which axis represents temp.? 3. where would blue stars be located?

Where in the graph are the bright, cool stars?
Which axis represents temperature? What area would blue stars be located?

*The more massive a star, the shorter its life will be.

Birth of a Star A star begins its life in a cloud of gas & dust called a Nebula. A nebula condenses when an outside force (shockwave) acts upon it. Parts of the nebula then condense due to gravity & the temp. increases. Large enough nebulas will begin to glow (Protostars) Protostars get hotter & brighter eventually fusion occurs & a star is born.

How would the amounts of H & He in the cores of 2 identical stars compare if one star was older than the other? Answer: the older star would have been fusing H into He for a longer period of time, so the older star would have more He in its core than the younger star.

Death of a star like the Sun (main sequence star)
Balance of fusion & gravity occurs until H is used up & the core shrinks & releases new heat that expands the star into a Giant. The star begins to die when temps rise & He (helium) fuses to heavier elements (C-O core) Gas layers are blown away leaving C-O core (white dwarf) w/halo glow= Planetary Nebula. Eventually gases dissipate into space & white dwarf is left.

Death of a Massive Star Same start as before, but fusion occurs until Fe (iron) nuclei form & star swells more than 100 times the diameter of our sun, becoming a Supergiant. Fe cores absorb energy & suddenly collapse sending shockwaves & outer layers out in a huge burst of light= Supernova and produces new elements. Left behind is either a Black Hole (if original star was 15 x or more massive than our sun) or a Neutron Star. Ask: Why does a supergiant form a supernova but a giant does not? (supergiant forms iron nuclei that absorb energy, giant does not.) Last supernova occurred in 1987 in the Large Magellanic Cloud (close galaxy to milky way) gave scientist a chance to observe & test hypothesis about supernovae.

Remnants of Massive Stars
After a massive star “goes supernova,” it leaves behind its core. Neutron Star: super dense, all e- are pushed into the nuclei they orbit, result is a dense mass of neutrons that are trillions times more dense than the sun. At first, it spins & emits pulsing beams of radio waves out (Pulsar). If star was at least 15 x more massive than the sun: Black Hole: Intense gravitational field that not even light can escape. Detected by strong sources of x-ray (given off by atoms being ripped apart by gravity) Mysterious “Dark Matter” believed to be a source of black holes. Believed to be present everywhere.

Life Stages of Stars: From Birth to Death

Self-Quiz What determines which life-cycle course a star takes?
How does cloud-like nebula form protostars? What process marks the transition from protostar to star? What remains balanced during the long period when a star’s H is fusing into He? At what point is this balance lost? 1. The initial mass of the star. 2. Outside forces cause compression in parts of nebula. Regions become denser & temps rise, they start to glow, forming a protostar. 3. Fusion. 4. Energy produced by fusion is balanced by force of gravity. 5. When the H in a star’s core is used up.

Galaxies Universe is everything that exist. Observable universe is everything that we can observe limited by age of universe & speed of light. If universe is 12 B Y O then light from objects more than 12 Billion light-years away has not had enough time to reach us yet. Astronomers are not sure how old the universe is, estimates range from 10 to 20 B Y O.

What Are Galaxies? Systems that contain millions or billions of stars.
Look like hazy patches of light in the sky. Estimated to be billion galaxies in observable universe. Most are millions of light years apart.

To what galaxy do we belong?
Milky Way Galaxy It’s a spiral galaxy (pinwheel-shaped) Diameter of about 100,000 light-years Our sun is about 26,000 light-years from it’s center When looking in the night sky, you can see the plane of the milky way disk. It looks like a milky or hazy line because there are so many stars. Hundreds of billions of stars in the milky way galaxy.

Types of galaxies Spiral: pinwheel-shaped; nuclei of stars bulging at center & wound spiral arms. Elliptical: nearly spherical to oval-shaped; stars concentrated at their centers, little interstellar gas & dust for new stars to form so few to no young stars. Irregular: irregular in shape; much smaller & fainter than other 2 galaxy types, stars spread unevenly.

Spiral Galaxy

Elliptical Galaxy

Irregular Galaxy

Spiral, Elliptical, & Irregular

Active Galaxies Emit more energy than their stars can give off.
Some emit radiation, or change largely in brightness in short periods of time. Possible powered by a supermassive black hole centers that jet out hot gas in opposite directions. Quasars: very distant, extremely luminous celestial object believed to be a type nuclei in an active galaxy.