... Q&A: Galaxies (cont'd)
|11. What are colliding
When two or more galaxies are close enough to each other,
gravitational forces will pull the galaxies toward each other. This gravitational
attraction increases as the galaxies travel toward each other. The galaxies
may pass by each other or collide. Two galaxies that are interacting or
colliding may be referred to as a pair, or one galaxy may be referred
to as a companion of the other.
The Hubble images below show how different colliding galaxies can look.
The appearance of an interacting system of galaxies depends on many
factors, including the stage of the interaction, the number of galaxies
involved in the interaction, their masses and types, how close they
are, and how they approach each other.
The Antennae galaxies (upper left) are an example of two spirals that
are in the process of colliding. We will not see the end result during
our lifetimes because this process takes hundreds of millions of years.
Sometimes smaller galaxies plunge into larger galaxies. This type of
collision produces a ripple effect, like a rock thrown into a pond.
The Cartwheel galaxy (upper right) is an example of this type of collision.
The outer ring of blue stars in this galaxy indicates a ripple of star
formation resulting from the collision.
Our Milky Way and Andromeda are two spiral galaxies that may eventually
collide (about 5 billion years in the future).
|12. What happens
when galaxies collide?
When galaxies collide, they experience a gravitational pull toward
each other. This gravitational pull distorts the shapes of the galaxies,
and can pull material from one galaxy to the other. In many cases, the
pull of gravity may result in the galaxies merging. The individual stars
within the galaxies do not collide since they are so far apart, relative
to their size.
Clouds of gas within the galaxies do collide, however. As a result,
large amounts of gas become concentrated in one or more areas of the
system. As the collision compresses the gas, the gas becomes dense.
The clouds of gas collapse under gravitational forces and form large
numbers of new stars. This rapid, short-lived episode of star formation
activity is referred to as a starburst. Intense starbursts can use up
nearly all of the available gas.
all interacting galaxies merge?
Interacting galaxies do not always merge together to
form a single object. Scientists think that some galaxies (such as M51
and its companion, NGC 5195) will simply pass by each other without merging.
Galaxies can also pass through each other without merging. The Cartwheel
galaxy is believed to be the aftermath of such a pass-through. Some galaxies
do merge. The Antennae are thought to represent a merger between two gas-rich
spiral galaxies. When the merger is complete, the Antennae may end up
looking like a single elliptical galaxy.
|14. Why do we study
distant galaxies, if they are faint and hard to observe?
When we study astronomical objects, we are actually
looking back in time. Light from the Sun takes eight minutes to reach
Earth. The light we see today from the next nearest star was emitted about
four years ago. Light from the nearest galaxy like our own, Andromeda,
takes over 2 million years to reach us. That is, we see Andromeda as it
appeared more than 2 million years ago. Observations of distant galaxies
show us what the universe looked like at an earlier time in the history
of the universe. By studying the properties of galaxies at different epochs,
we can map the evolution of the universe.
|15. When scientists
study these distant galaxies, what do they look at?
They observe many properties of each galaxy, including
size, shape, brightness, color, amount of star formation, and distance
from Earth. This information helps astronomers to determine how these
structures may have formed and evolved.
|16. What is a "deep"
In astronomical terms, a deep field is a long-exposure observation taken to view very faint objects. Light from these objects is collected over a large period of time, so the detectors have a chance to gather as much light as possible. Objects can be very far away and appear faint to us due to the vast distances over which the light must travel; and/or objects can lie close to us and be faint because they don't give off much light. So "deep" doesn't necessarily mean far. However, in the case of the Hubble Deep Fields (HDFs) and the Hubble Ultra Deep Field (HUDF), deep does mean far away since the images were taken in areas that we know have few nearby stars.