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Part 3: Characteristics of Stars

STARS AND GALAXIES

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Basic Characteristics

• Composition (Elements made of)

• Color and Temperature

• Brightness

• Size

• Distance from Earth

• Mass

Brightness and color can be used to determine many of the other characteristics.

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• If you heat an element until it gives off light, the light given off will have a characteristic spectrum revealed by a spectroscope.

• Video: The Temperature and Composition of Stars

Spectra of Elements

HydrogenHeliumCarbon

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• The spectrum of a star tells us the composition of a star (the elements it is made of.)

• The black lines in the spectrum are absorption lines, and they can be used to determine composition of the star by comparing their location to the known spectra of elements.

Composition

Spectrum of a star. Graph of spectral luminosity versus wavelength.

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• Most stars are composed predominately of hydrogen, the lightest and most basic element in the universe.

• Helium is the second most common element in a typical star.

• Hydrogen and Helium = 96-99% of a star’s mass.

• Other elements often include oxygen, neon, carbon, and nitrogen and even heavier elements such as calcium and iron.

Composition

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• While our Sun is yellow, stars come in many colors.

• Astronomers study a star's spectrum to determine the dominant color of the star.

• Recall that the color of light is directly related to its energy:

Color and Temperature

Lower EnergyCooler

Higher EnergyHotter

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• The color of a star indicates its surface temperature.

• The sun’s surface temperature is about 5,800 K, while its core temperature is as high as 15,000,000 K.

• Red stars are 3,000 K or less, while blue stars can be up to 60,000 K.

Color and Temperature

O – Blue

B – Blue/W

hite

A – W

hite

F - W

hite/Ye

llow

G – Ye

llow

K – Oran

ge

M - Red

0

10000

20000

30000

40000

50000

60000

70000

Temperature Ranges for Spectral Classes of Stars

LowHigh

Spectral Class

Surf

ace

Tem

pera

ture

(K)

Review: K is Kelvin and 0°C = 273 K

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Depends upon three factors:

1. The temperature of the star

2. The star’s size

3. The star’s distance from Earth

• Apparent magnitude: the brightness of a star as it appears on Earth

• Absolute magnitude: the amount of light actually given off by a star; calculated by astronomers

Brightness of Stars

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• Brightness is constant for most stars.

• Some stars vary in brightness and are called variable stars.

• A star may be variable because something blocks light from reaching the Earth, as with the binary star Algol, or because the light being emitted varies.

Brightness of Stars – Variable Stars

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• A star that varies in both brightness and size in a regular cycle is called a pulsating variable star.

• The North Star (Polaris) is an example of a Cepheid variable, one kind of pulsating variable star. FYI: Polaris dims and brightens in a 4-day cycle, but not enough to detect with the naked eye.

Brightness of Stars – Variable Stars

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In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell, working independently, found that as the absolute magnitude of stars increases, the temperature usually also increases.

Hertzsprung-Russell (H-R) Diagram

Idealized graphic, not the plot of actual stars

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• Remember color indicates surface temperature

• Plotting magnitude vs. color reveals that most stars fall within several groups.

• Main sequence stars (90%) are hydrogen-burning stars

Hertzsprung-Russell (H-R) Diagram

Idealized graphic, not the plot of actual stars

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• (Red) Giants and Supergiants – generally much larger than main sequence stars

• These stars have begun to die. They have depleted their hydrogen and now burn heavier elements.

Hertzsprung-Russell (H-R) Diagram

Idealized graphic, not the plot of actual stars

Supergiants: Rigel (blue), Betelgeuse, AntaresGiant Star: Aldebaran

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• White Dwarfs – as a red giant continues to die it eventually transforms into a white dwarf

• A typical white dwarf is about as massive as the Sun, yet only slightly bigger than the Earth.

• A white dwarf will continue to lose energy until it is a non-emitting ball of gas, a black dwarf.

Hertzsprung-Russell (H-R) Diagram

Idealized graphic, not the plot of actual stars

Example: Sirius B, part of the Sirius Binary System, brightest star in the sky

Hertzsprung-Russell Diagram

From atlasoftheuniverse.com via Wikimedia Commons

Diagram with 23,000 stars plotted.

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• Stars vary tremendously in size

• The Sun, with a diameter of 1,392,000km, is a medium size star.

• Stars from 1/10th to 10 times the size of the Sun are considered medium-sized and this includes most stars.

So How Large Are Stars?

Scientific Notation: 1,392,000km = 1.392 X 106km or 1.392E6

 The Sun photographed by NASA's Solar Dynamics

Observatory (SDO). This is a false color image of the sun observed in the extreme ultraviolet region

of the spectrum. 

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• 16-20 km in diameter

• Mass of about 1.4 times that of our Sun.

• A neutron star is so dense that on Earth, one teaspoonful would weigh a 100 million tons!

• Like a white dwarf, a neutron star is the remains of a dying star, but one that exploded in a supernova.

The Smallest Star – A Neutron Star

The first direct observation of a neutron star in visible light.

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• Most commonly measured in light years. (Remember: a light year is the distance light can travel in one year.  It is equal to 9.46 x 1012 km.)

• How we calculate the distance to a star depends upon how far away the star is.

Measuring Star Distance

Parallax

TRY IT: If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift" with respect to more distant objects behind it. This is because your eyes are separated from each other by a few inches - so each eye sees the finger in front of you from a slightly different angle. The amount your finger seems to shift is called its "parallax". (http://

heasarc.nasa.gov/docs/cosmic/nearest_star_info.html)

What happens to the amountof shift as the object being observed moves further away?

Note: The diagram to the right is exaggerated for teaching purposes.

• The distance the Earth travels is very small compared to the distance to the star, so the parallax angle (shift) is very, very small, even for close stars.

• Using that angle, one can calculate the distance to the star.

Stellar Parallax

Diagram from Windows to the Universe: http://www.windows2universe.org/kids_space/star_dist.html

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• Because the parallax angle gets increasingly small the further away the star is, parallax can only be used to calculate the distance to stars closer than 400 light-years away, and is most accurate for close stars.

• FYI: Hipparcos was a satellite that operated between 1989 and 1993. Its purpose was the accurate measurement of the positions of celestial objects. This permitted the relatively accurate determination of distance to stars up to 400 light years away using parallax measurements (versus 100 light years using measurements made on Earth).

Stellar Parallax

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• For stars beyond 400 light years away, astronomers use a comparison between absolute magnitude and apparent magnitude to estimate distance.

• What do you think is the biggest challenge to doing this?

Beyond 400 Light Years

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• In the core of a star, the forces of gravity are extremely strong.

• As matter is crunched together tighter and tighter by gravity it heats up to millions of Kelvins.

Why Stars Shine

http://www.swpc.noaa.gov/primer/primer.html

FYI: In our Sun, 4.2 billion kg of mass are converted to energy each second.

• This ignites the process of nuclear fusion, causing hydrogen atoms to fuse together.

• As atoms are fused, some of the matter is converted energy and given off as electromagnetic waves.