The Universe of Galaxies
- Galaxies move and sometimes
collide
with each other.
- While galaxies do move relative to each other, there is an additional
apparent motion: the entire universe is expanding.
- Essentially all galaxies we see are moving away from us.
- The speed at which they are moving away is proportional to their
distance; more distant galaxies appear to be moving away faster than
closer galaxies. In this situation, no matter what galaxy we lived in,
we would see that all other galaxies would be moving away from us.
- Consequently, we believe the entire Universe is expanding, most
likely without any center of the expansion.
- Even though the Universe as a whole is expanding, small regions
of the Universe where there are objects (groups or clusters of galaxies)
can be contracting under the force of
gravity
- Given the expansion rate, we can estimate how long ago the Universe
would have been at a single point, and we find that this occurred 10 to
20 billion years ago, assuming that the universe has always been
expanding.
- The model/theory that the Universe was originally much more concentrated
and that there was a time when everything was collapsed together is
called the Big Bang theory
- The Big Bang theory was motivated by the observation that
all galaxies appear to be moving apart from each other. However, this
observation alone leaves open the question about whether the Universe
has always been expanding.
- The Big Bang theory got a major boost with the discovery
several decades
ago of the microwave background radiation; the entire
sky is glowing very faintly in a kind of light called microwave
light. This was predicted as a result of the Universe having
been hotter when it was denser; in the microwave background, we
are seeing the glow of the hot universe long ago.
- this is a good example of how science works: observations
(the recession of galaxies) motivated a model (the expanding universe),
which made a prediction (a glowing early Universe) that was verified
by observations (the microwave background).
- The Big Bang theory is also supported by our current best understanding
of basic physics.
- We can measure the expansion rate from a long time ago by looking
at very distant galaxies (since we are looking back in time to see
these). Recent observations, surprisingly, suggest that the expansion
rate is not slowing down as one would expect. This
has led to the idea that there is something in the Universe
that is causing it to accelerate. We don't know what this
is, astronomers have dubbed it
``dark energy''
- Note that this dark energy is not to be confused with ``dark
matter'', which also exists and which we will discuss in a few
weeks!
Distances in astronomy - how do we know how far away things are?
- One common theme we've seen throughout our "Overview of the Universe"
is the recognition of how far apart things are in the Universe. This raises
the question: how do you measure distances to things that are so far away?
- We are used to measuring distances directly, e.g., with a ruler.
- Direct measurements in astronomy are generally not possible,
although a form of them using light travel time (laser ranging),
can be used for some objects within the solar system. (For example,
the APOLLO
experiment
at NMSU's Apache Point Observatory!
- Distances is astronomy can be tricky to measure, especially for very
distant objects, and determining distances to the most distant objects
involves several steps.
- Measuring distances with angles
- Measuring the apparent size of an object gives its distance
if we know its true size. The apparent size will be smaller if
the object if farther away. The relationship is simple:
apparent size
If we can measure the apparent size, and
know what the true size is, we can determine its distance.
- When we talk about the apparent size of an object, we
are talking about the angle that it subtends in our view
- For nearby astronomical objects, we can measure distances through
a geometric technique known as parallax, which is the same technique used by
surveyors on Earth. It basically uses the apparent size argument in
reverse: instead of looking at an object of known size from one vantage
point, it looks at the point from the two ends of an object of known size,
or in other words, from two points where we know the distance between
them. This involves observing an object from two different
vantage points, and measuring how the direction we need to look changes from
one vantage point to another. The amount the direction changes
gets smaller as objects get farther away.
- The farther apart the vantage points, the more
the direction will appear to change.
- This means for a fixed accuracy of measuring angles, we can measure
more distant objects if we use a wider baseline (vantage points that
are further apart).
- To measure the distances to stars, which are very far away, we
use the widest baseline we can: namely, the Earth's position at two
opposite points on its orbit!
- However, even from this wide baseline, the apparent motions that
we use to measure distances are only big enough for a few thousand
stars in the neighborhood of the Sun in our Milky Way!
- The beauty of parallax is that you measure distances using simple
geometry. You don't need to know anything about the object that you're
looking at, you just need to be able to see it from two different
vantage points.
- For more distant objects, we can measure distances by using
brightnesses of objects, since objects will appear fainter if they
are at larger distances. However, this is trickier, because not all
astronomical objects have the same intrinsic brightness, so
when you see an object that appears faint, you don't immediately know
whether it is closer and intrinsically faint, or farther and intrinisically
brighter.
- The apparent brightness of objects depends on the distance:
more distant objects appear fainter than identical nearby object.
- Measuring the apparent brightness of an object gives its
distance if we know its true brightness. The apparent brightness
of an object is fainter if it is farther away.
apparent brightness
This is known as the inverse square law of apparent brightness.
- The true brightness is a measure of how much light (how
many ``light arrows'' are coming from a source); it is also known as
the luminosity. The apparent brightness is a measure
of how much light is received at some location (how many ``light arrows''
are collected in your ``light bucket''); it is also known as the
flux.
- If we can measure the apparent brightness, and know what the true
brightness is, we can determine its distance.
- We can sometimes know what the true brightness is by studying the type
of light that stars produce.
Some examples of objects used as standard candles, that
is, objects with known intrinsic brightness are: a kind of variable
star called a Cepheid variable, and a particular kind of supernova
explosion.
This technique can work out to large distances.
- By the time we talk about measuring distances to the most distant
objects, we are depending on several previous measurements of distances
to closer objects. As a result, distances become more and more uncertain
as they become larger. Still, it is quite amazing the we believe that we
know distances to astronomical objects, even those at the largest distances,
to a reasonable level of accuracy: the distances we measure may be off by
a few percent, or even a few tens of percent for the most distant objects,
but they are not off by much more than this!
- Since distances in astronomy are so large,
they're difficult to comprehend if we use normal measures of distance
such as the mile or kilometer. Astronomers often use different units
to measure distances so the numbers are not so gigantic.
- Inside the Solar System, differences are often measured in a unit
called the astronomical unit. One astronomical unit is defined
as the average distance between the Earth and the Sun. Pluto is about
40 astronomical units away from the Sun.
- For larger distances, astronomers often measure distances using
light travel time. For distant objects, light takes a significant
amount of time to get to us on Earth, so we can, for example, use a
unit called the light year, which is another measure of
distance. A light year is the distance than light travels in one
year; it is a long way, as light travels at 186,000 miles per second!
The Sun is about 7 light minutes away, Pluto is several light hours
away, the nearest star is several light years away, the Galaxy is
thousands of light years across, and other galaxies are millions to
billions of light years away.
- Astronomers also use a distance unit called a parsec, or a
kiloparsec (1000 parsecs); we won't go into the details here, but a
parsec is about 3.25 light years. The units of parsecs come directly from
the geometric technique of parallax.
- Review of the objects in the Universe, with rough size scales
- solar system: light-hours across
- asteroids
- comets
- dwarf planets
- inner planets
- outer planets
- Sun: light minutes away from Earth
- nearby stars: light-years away
- center of the Milky Way: tens of thousands of light years away
- Milky way galaxy: hundred thousands of light years across
- nearest galaxes: millions of light years away
- distant galaxies: billions of light years away
- A nice summary of the range of distances in astronomy is given
by the (relatively old) short video, Powers of 10, which is available on
Youtube (link
Here is a more recent animation
of where we fit into the Universe
- Since we see objects so far away, we are seeing what they looked like
in the past, and this demonstrates that the Universe has been around for
a long time.
Next: PART 4 - THE
Up: AY110 class notes
Previous: PART 2 - MOTIONS
Jon Holtzman
2013-12-06