How do you know that?

by Paul Doherty

When you look at a Hubble photograph you see colorful patterns.

Scientists extract information from these patterns.

For stars, scientists learn the:

What is it in the photographs that tell the scientists these stellar properties?

Apparent Brightness

The apparent brightness of a star is how bright it appears on earth. The ancient Greeks devised a system in which the stars in the sky were divided into groups. The brightest stars were in the first group, the next dimmer stars in the second group, the dimmest stars in the sixth group. This system has evolved into the system of stellar magnitudes were use today. Brightest stars are first magnitude, followed by the dimmer second magnitude and so on.

The Greeks used their eyes to estimate the magnitudes of stars, later people used photographic film, now we use electronic detectors.

The Hubble telescope, like most modern telescopes, makes its images using CCD's. Charge Coupled Devices. CCD's are also used in personal digital cameras and video cameras. These semiconductor devices have millions of picture elements, pixels arranged in a square. When a photon hits a pixel the semiconductor absorbs the photon and uses the energy to release an electron. The freed electrons are stored in the pixel until a signal arrives to read-out that pixel. Then all the electrons in the pixel are counted as they are released.

Brighter stars deliver more photons to the pixels of the CCD's than dimmer stars. You can measure the apparent brightness of a star by counting the electrons in the CCD. Of course this does not tell you the actual brightness of the star, its absolute magnitude or its luminosity unless you also know the distance to the star. Because distant objects appear dimmer due to the inverse square law.


Red hot, white hot, our language expresses temperature in terms of color. Turn on an electric heating element on a stove n a dark room. At first you can feel the infrared radiation from the burner as it strikes your flesh.(the backs of your hands and your cheeks are particularly sensitive.) Yet you detect nothing with your eyes. Then as it heats up the burner begins to emit a dim dark red light. Warming further the light becomes brighter red and then orange. If yo could heat it more the light would become the yellow-white of a lamp filament and then the white of the sun.

Look up at the stars. Mostly you don't see any color because the stars are too dim to trigger the color sensitive cones of your eye. But look at the brighter stars like Betelgeuse in the shoulder of Orion and you will see that it is red, or Rigel in Orion's opposite knee and you will see that it is bluish white. Look through a telescope and stars come alive with color. The telescope is a light bucket, the mirror or lens catches light and squeezes it down through the tiny pupil of your eye making all the stars brighter.

Look at the constellation Cygnus the Swan through a telescope. The "star that passed over the fence last" the ass of the swan, is Deneb. The telescope shows it is a double star one star of the double is blue, the other is yellow. The blue star is hotter than the yellow star.

Scientists measure the temperature of the surface of a star by looking at its color. Just like the human eye measure color by measuring the intensity of red, green and blue portions of the spectrum then comparing these intensities, astronomers photograph stars through different filters. These color filter pictures of stars tell the astronomer the temperature of the star. These multi exposure photographs through filters also allow scientists to make color, and false color images of stars, nebula and galaxies that look so beautiful in the galleries of Hubble images.

The exhibit and snack Blow Out Your Toaster can be used to heat nichrome wire to different temperatures to observe its color.


What is it made of?

Heat up a dilute gas and it will glow. Look at the gas through a spectroscope and the glow will be separated into lines. Each element produces its own pattern of lines, its spectral fingerprint. So take the spectrum of a nebula and you will see the spectral lines of the elements which make it.

Helium was discovered in the sun before it was discovered on earth. The spectrum of the sun had lines no one had ever seen from any element on earth. later helium was discovered on earth when it was found to have the same spectrum as the unknown gas on the sun.

The exhibit/snack spectra will give you a good feel for how elements are identified.

The result is interesting, the universe is made of 75% hydrogen and 25 % helium by mass. The rest of the elements are just impurities.


To near stars

The distance to nearby stars is done by trigonometry.

Look at a star compared to distant background stars when the earth is on one side of the sun, then look at the same star again six months later when the earth is on the other side of the sun and you can use trigonometry to figure out the distance to the star.

You can do a simple version of this experiment by pointing a finger at a distant object.

Then while holding your finger still, close your right eye, then open your right eye and close your left eye. Repeat. You will see your finger jump back and forth from. Your finger is closer to your eye than the background and so jumps back and forth as your point of view changes.

See the activity Master Eye.

Recently the Hipparcos satellite took trigonometric measurements of stars above the obscuring atmosphere of earth and made the best measurements of stellar distances ever. These distances included stars ten times further away than had ever been measured before. Most importantly they included measurements of the distances to the first Cepheid variables.

To near galaxies

Distances to nearby galaxies are measured using Cepheid variable stars.

Henrietta Leavitt at Harvard discovered a type of variable star in the Magellanic clouds.
A Cepheid is a variable star whose intrinsic brightness is related to the period of its oscillations. Cepheid variable stars that oscillate slowly are larger and also brighter stars.

Measure the length of the period of a Cepheid variable and you know how bright it actually is. Measure how bright it appears from earth and then you can find how far away it is. (Hint: use the inverse square law for brightness.)

The Hubble telescope allows us to measure the period and apparent brightness of Cepheids in galaxies 100 million light years away.

To far galaxies

Larger stars end their lives as white dwarfs. A white dwarf that accrues mass over 1.4 solar masses explodes as a type 1a supernova, one of the brightest objects in the universe.(The white dwarf may gather mass ejected by a companion star that is swelling into a red giant. The 1.4 solar mass limit is called the Chandresehkar limit.)

The Hubble telescope can see type 1a supernovas billions of light years away. Scientists believe that all type 1a supernovas have almost the same maximum brightness. They also believe that the curve of brightness will tell them whether each supernova is average,or brighter or dimmer than average. Once again if we know the actual brightness of an object and its measured brightness on earth we can find its distance.

You can find the distance to the sun if you know how bright it is (we do) and compare it to the brightness of a lightbulb.

The distance to the sun


When an object emits light as it moves away from us, the lines in its spectrum emitted by elements like hydrogen, are Doppler shifted into the red. There is a spectrograph on the Hubble telescope which records the spectrum of light from hot elements in stars, nebula and galaxies and reveals their Doppler shift. The Doppler shift can be converted into the speed of recession of the object emitting the light.

Astronomers customarily divide the Doppler shift of distant galaxy clusters into two parts. the cosmological red shift due to the expansion of space between us and it, and the Doppler shift due to the motion of the galaxy we are looking at with respect to the local space through which it is moving.

The Andromeda nebula, the nearest big galaxy to us is actually moving toward us and so its light is blue shifted.

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Scientific Explorations with Paul Doherty

© 2001

7 March 2001