What Do You See?

by Paul Doherty

When you look at a photograph from the Hubble Space Telescope what do you see?

Star dots

Almost every star is too small to be spread over more than one pixel on the Hubble telescope CCDs. One exception is Betelgeuse, the red supergiant star in the shoulder of the constellation of Orion the Hunter. Betelgeuse is so big that if it were placed where our sun is, its visible surface would reach out beyond Jupiter! It is so big that the Hubble space telescope can resolve it into a stellar image.

Stars are huge objects yet the distances between them are so immense that even in the best telescopes they appear as dots.


Notice the crosses through stars in most photographs.
Each straight line is due to diffraction from a support structure in the tube of the telescope. Every bit of light produces such a cross, but only the brightest stars make diffraction patterns that are bright enough to be recorded by the CCD's of the telescope.

When you see a cross on a Hubble space telescope image you can be sure it marks a bright star.


The space telescope counts electrons, and electrons have no color, so the cameras on the telescope take photos in shades of gray. However scientists can take several gray photos through different color filters then print photos with these colors added to the gray to produce color photographs. If they use red green ad blue filters to take the photographs and to print them then they get a photo of how the object might look to a human eye transported to a place near the object. (The human eye would have to be near to get enough photons to make color vision work.)

However scientists are tricky, they can use filters on the Hubble telescope that pass infrared or ultraviolet light which would be invisible to human eyes. They can then print the photos so that infrared appears red and ultraviolet appears blue. These images are full of information but they are not true color images, they are false color images.


Unlike stars, galaxies appear as recognizable shapes in Hubble telescope images. On the average, galaxies contain a hundred billion stars. (1011)

What you see in a galaxy is: stars and gas and dust. The stars glow with light powered by nuclear fusion in their cores. The gas glows when excited by starlight and the dust reflects starlight in which case it appears white or blocks it appearing dark.

There are three basic families of galaxy shapes: spiral, elliptical and irregular.

Spiral galaxies look like pinwheels. They have curving arms, called spiral arms, wound around their center. Often there is a spherical central bulge of stars. Our Milky Way galaxy is a spiral galaxy. The spiral arms are waves of star formation swirling around the galaxy. The new young stars are hot and bright, they light up the gas and dust in the arms. Some spirals have a bright bar across their central regions, these are called barred spirals.

Elliptical galaxies are vast ellipsoids of stars. Some are spheres.

Irregular galaxies are, well, irregular. In the Hubble deep field photograph there appear to be many more irregular galaxies than there are near us today, perhaps these photographs into the deep past show us galaxies forming. Early galaxies that had not evolved to their mature shapes.

Some photos show galaxies interacting with each other. We know that the Andromeda galaxy will collide with our Milky Way galaxy in a hundred million years or so. Stars in galaxies are so spread out that two galaxies with a hundred billion stars each can pass through each other without any stars actually colliding! However the gravitational effects of each galaxy on the other disrupts the orbits of the stars. We call these gravitational effects tides. And see amazing images of galaxies passing through each other and ripping each other apart strewing vast streams of stars across the sky.

The centers of the most distant galaxies seem to be very energetic. They spew out vast filaments of gas at a good fraction of the speed of light. Radio telescopes first showed these jets, now we can see them in Hubble telescope images. One hypothesis is that these jets are spit out by a supermassive black hole at the center of the galaxy.


Some of the most beautiful colored patterns in the Hubble telescope images are the nebulas. Glowing clouds of gas and dust powered by stars.

Let's divide nebulas into two types: those associated with star birth and those with star death.

Star Birth

In the constellation of Orion there is a three "star" dagger hanging down from the three star line of his belt. The middle "star" of the dagger is not a star at all, it is the Orion Nebula. Near the center of the nebula there are four bright stars arranged in a trapezoid and called "the trapezium." These stars are hot and pour out ultraviolet light. Ultraviolet light not only makes gas fluoresce it pushes on gas and dust. (Actually all wavelengths of light exert pressure.) Looking at the images surrounding the trapezium we see an array of sperm like streamlined globs all with their fat heads pointing toward the trapezium. The less dense gas surrounding these denser regions has been blown away by starlight leaving these streamlined shapes behind. These shapes are the birthplace of stars. Sometimes near their centers dark dust lanes split glowing clouds. The glowing clouds are illuminated by the birth of a star, the dust lane is in the plane of the stars equator and may be a place where planets form. Thus these shapes were given the name proplyds, for proto-planetary disks. Proplyds were discovered in Hubble telescope images.

Star Death

Often a nebula will appear as a ring. Look closely at the center of the ring and there is a bright blue dot. Such rings are called planetary nebula since researchers once guessed they might be associated with the birth of solar systems. Now however we know the mark the old age of stars. When stars age they blow off their outer parts in a giant solar wind. This wind can blow a spherical bubble. When we look at the sphere all we can see are its edges so it looks like a circle.


The colors of the nebulas are the colors of the spectra of the gasses which make them up. Each color represents a different gas or mixture of gasses. Hydrogen appears as red, oxygen as green.


Occasionally one star will shine as bright as the hundred billion other stars in a galaxy. These bright stars are called supernovae. Thype II supernovae are the result of the explosive death of stars much more massive than the sun. Type 1 supernovae are the result of the death of a white dwarf star which has accumulated some mass from it neighbor and grown beyond the Chandresehkar limit of 1.4 solar masses where the laws of quantum mechanics and relativit declare that it must collapse to form a neutron star.

Because all type 1supernova start with the same mass they reach nearly the same brightness. This uniform brightness can be used to measure the distance to the supernova and therfore to the galaxy which houses it.

The supernova eject shells og gas that make rings around the star. For nearby supernova, the chaging shape of these rings can be followed with the hubble telescope.


The Crab Nebula in the constellation of Taurus the Bull was the result of a supernova explosion in 1054 AD. The nebula changes so rapidly that there is a movie of its motion compiled from hubble telescope images.

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Scientific Explorations with Paul Doherty

© 2001

7 March 2001