- There are some intimate connections between astronomy and our
existence. A brief history
of the universe:
- About 13 billion years ago, the Universe came into existence - don't
know how or why.
- Amazingly, we know something about its state in its youth. It
was very different: hot and dense and made of light
- In the first few minutes, the pure energy was transformed into
the basic particles we see today: protons, neutrons, and electrons.
However, only the simplest elements were made: hydrogen, helium, and a
tiny bit of lithium and beryllium
- As time passed, the Universe expanded rapidly and cooled off
- After a few million years, the matter in the universe which was
smoothly distributed collapsed into blobs under the force of gravity.
At some point, in the centers of these blobs, stars were formed.
- The stars and gas around them separated into distinct groups in
space which we now call galaxies
- Each star in each galaxy acted as a nuclear reactor, converting simple
elements into more complex ones and producing light in the process
- Some of the stars exploded, sending the more complex elements back
into the gas, from which new stars were formed. Around some of these
stars, some of the complex elements condensed into rocky balls which
we call planets.
- About 5 billion years ago, a star was formed in the outer region
of one of the galaxies. Around this star, a system of nine planets
formed. The size and temperature of the 3rd planet from the star was
such that liquid water existed on its surface.
- On this planet, the combination of elements along with the light
from the star allowed chemical reactions to take place which allowed
the formation of simple life. The main elements in most life forms included
some of the elements that were synthesized in previous generations of
stars: carbon, nitrogen, and oxygen.
- Over several billion years, the life forms changed and evolved. Very
recently, only a few million years ago, humans appeared on the planet.
They lived simply for millions of years. They probably looked at the
stars and wondered about them, not dreaming that their existence
depended on previous generations of stars. Connection 1:
Humans are made partially of stardust.
- Astronomy has also been used practically by humans. Very recently
(on the astronomical time scale), humans developed agriculture and
depended on their observations of the sky to help them determine
when to plant crops. Other cultures used observations of the night
sky for navigation. Connection 2: Astronomy played a
role in the development of modern civilization.
- Humans are part of astronomy. We send spacecraft from the
Earth to other places, and have an effect on the global properties
of our own planet, Earth. Connection 3: By our
existence, we affecting astronomical objects, at least within
the Solar System, and espeically, the Earth.
- Recent advances in technological tools make today a special time;
astronomy is at a very exciting stage! This technology allows us to
see much more of the Universe than we can by eye, and our view of
the Universe has changed dramatically.
((pictures in slideshow form))
- Where Earth was thought to be large and immutable, changing gradually,
if at all, we now have seen Earth from space and realized how small
and fragile it is in the overall scheme of things.
We know that the very
continents
move around and collide with each other.
It's possible that the history of Earth is dramatically influenced
by the
impact of comets
on its surface.
- Where people once saw the Sun an unchanging, we now know it is
quite dynamic
- Where people once saw planets as bright points of light, we now
see them as
worlds of their own,
with
rapidly changing atmospheres
and systems of moons and rings around them. Some of these worlds have
active geology
and perhaps even harbor some form of life.
- Where people once saw unchanging stars and planets, we now see
material between stars,
newly forming stars, and
exploding stars
- People now see
galaxies
and can measure their motion through space and understand that the Universe
itself is expanding.
- We realize that there are astrophysical phenomena operating on
all time scales, from the slow evolution of stars and galaxies on
timescales of billions of years, to supernove and gamma ray bursts on
the timescales of days and seconds.
- New technologies of
big telescopes
and
telescopes in space
allow
us to look far back into time and see what the Universe looked like
long ago.
- There are lots of recent astronomical developments
- The development of a fundamental understanding of the universe
is still in progress. Just in the past few years, astronomers have
revised their ideas about the overall shape, age, and contents of
the Universe. In the current picture, large fractions of the Universe
are thought to be in some still-unknown forms of ``dark matter''
and ``dark energy''.
- Although we think we know a lot, there's a good chance we don't.
Mankind has been around for only a tiny fraction of the time the
Universe has been around, and the technological age has been only
a tiny fraction of that. If the history of the universe were placed
into the framework of a week, mankind would have appeared about a
minute before midnight on the last day of the week, and the
technological age just a fraction of a second before midnight.
- Astronomy is a crossroads science, involving geology, physics, chemistry,
and mathematics
- Astronomy is trickier than other sciences because, for many objects,
we can't visit what we study. However, the vast distances involved give us
a unique capability - the ability to look into the past!
- New Mexico is a great place
to look at the sky!
What are the goals for this class?
- Communicate to you how we have come to understand something about the
Universe, and to give you an appreciation of the Universe and of the
exceptional ability of humans to learn about it
- Understand that science is a process, not a collection of facts.
- Questions are more important than answers
- Try to formulate class around questions, rather than rote recital of
facts. See the class outline.
- Understand what a scientific result is, and how it differs from
an opinion
- Develop a broad appreciation for science & its impact on society.
What good does studying astronomy do for you?
- Helps you in the process of logical thinking - look critically at
what you are told and assess for yourself whether it is reasonable.
- Helps you appreciate the world aesthetically, if you can understand
why things appear and behave as they do
- Improve the condition of the world by acting on your knowledge and
understanding of the scientific process
- Get some enjoyment out of looking at the sky and imagining some of the
vast distances and incredible events that are going on out there
Class logistics
How does research astronomy, and more generally, science, work?
- There is a distinction between observational data and theories that
attempt to explain them.
- Data are determined from observations. Sometimes, the process of
obtaining, or interpreting, data can be tricky. An interesting part of
the scientific process is understanding how we know certain things.
For example, consider the following facts, and think about how we know them:
the Earth is round, the Earth goes around the Sun, the Moon goes around
the Earth. How do we know? Science involves formulating critical
questions to determine whether ideas are in fact true.
- What is the difference between a theory and a scientific theory?
- It is usually very difficult to prove that something is correct. Most
of the time, scienctists spend time trying to prove that theories and ``facts''
suggested by other people are wrong! This is a hallmark of a scientific
theory
- In general, a succesful theory not only needs to provide a plausible
explanation of a fact, it generally needs to make predictions for new
observations that can be used to verify it. Theories are often also judged
by their level of simplicity, i.e. a simple theory that explains a series
of observations is usually preferred to a more complex one that makes no
better or additional predictions.
- In this class, as in science, a fact or theory means something that so far
no one has been able to prove wrong; however, people have generally tried
very hard to do so!
- Skepticism is an important part of the scientific process.
``Qualification'' of opinions is a trademark of scientists, but can
be misinterpreted by non-scientists.
- When judging the validity of scientific results, one might consider
the information, attempts to invalidate the information, the sources of
information, and possible biases of the investigators.
- Much of the ``art'' of science is in coming up with the ideas, or
hypotheses, to test. In research, the initial hypotheses are generated by
scientists for any of a number of reasons. In astronomy, many hypotheses
are generated by looking at pictures and trying to figure out what might
explain the way things look.
- We will try to concentrate on this class on understanding not just
what we know about astronomy, but on how we know it, and also, on what
we don't know; at an introductory level, this can be challenging, however.
What is the difference between astronomy and astrology?
- Class query:
- How many people know the difference between astronomy and astrology?
- How many people believe in astrology?
- How many people read horoscopes?
- Astrology is something which purports that the position of the planets
and the stars at the time of your birth determines the course of your
future life.
- Is astrology a science? There is nothing about this hypothesis
which is directly non-scientific; scientists are free to come up with
whatever crazy theories they want. However, the process of science is
asking questions about hypotheses, so any good scientist will
ask the questions: does the hypothesis of astrology work? Is there any
known physical mechanism by which we expect astrology to work?
- There is no physical mechanism for astrology to work. It turns out
that all of the phenomena that we see in the universe around us can be
explained with only four basic forces: gravity, electromagnetism, and
two nuclear forces known as the strong and weak forces. Via these forces,
the location of planets and stars at the moment of birth are not relevant,
and besides the whole idea is a bit strange since we know that humans
develop over a nine month period before they are born.
- There is no evidence that astrology actually works. Note that the
predictions of astrology may work sometimes; almost certainly,
some of these predictions will work sometimes by chance! But for astrology
to be a legitimate science, it has to work all of the time.
Gravity doesn't just work ''sometimes''; it's the law! Here is a
link to some studies
on the predictions of astrology.
- Certainly, the widespread practice of astrology, namely horoscopes, is
incredibly suspect. One only needs to look at the
different horoscopes for a given day
predicted by a variety of astrologers!
- Does astrology have the right to be respected as a religion? If so, it
is a fairly shallow religion with no moral guidelines, as it believes things
are predetermined.
- Astrology is an examle of what is known as a pseudo-science,
or a superstition. Is it ok to regard pseudo-sciences are harmless
entertainment? Maybe, depends how far you take them. When elected
officials consult astrologers, I think there is some cause for concern.
- This is a real issue. As the world becomes increasingly technologically
sophisticated, it seems that more people are looking for ``simplicity'',
and have difficulties assessing the validity of the different things they
are told. However, there is a real and fundamental difference between
a science and a pseudo-science. This lies in the degree of scrutiny to
which hypotheses are subjected. In sciences, people try hard to
disprove theories.
- This is not to say, however, that even scientific results cannot
be manipulated. It is a good skill to be critical!
What do astronomers do?
- Astronomy is the science of studying objects seen in the
sky. Astronomers try to understand the nature of astronomical objects,
figuring out what is out there, details about the physical nature of the
objects, and try to develop an understanding of why the objects look
what they look like, how they got there, and how they might change
with time.
- Jobs: astronomy doesn't have too many immediate applications which provide
profit, so many astronomers are hired by the public sector: universities
and government research centers. However, increasingly, astronomical
missions are being built by the private sector, so there's a growing
number of astronomers hired by aerospace industry. Astronomy/physics training,
however, seems to be in moderately high demand, for a variety of
apparently unrelated fields, e.g. on Wall Street!
- Research. Generally, there are three main types of research
activity that astronomers might do.
- observational research: collection and analyze data, namely light,
from astronomical objects. Several stages:
- Collection of light usually requires staying up at night at a
telescope. However, in this day and age, the analysis of the data takes a
lot longer than the collection, so even purely observational astronomers
don't observe that many nights; typically, an astronomer might spend a few
to a dozen nights per year at the telescope. These days, most information
is recorded electronically, and observing at the telescope usually means
sitting in a warm room controlling the telescope and instrument with a
computer, looking at the data as it comes in to judge whether more data
is needed, listening to music, eating cookies, etc.
- The process of data reduction comes next; in this process,
astronomers correct for all of the features that are introduced by the
telescope and the Earth's atmosphere.
- Astronomers try to interpret what they are seeing in the light of
some model of what is going on in their objects, or try to come up with
some model of what is going on which other scientists can try to shoot
down! Computers usually play a large role in astronomical research
these days, as single pictures consist of LOTS of individual pieces
of information.
- theoretical research, where astronomers try to predict what objects
in the sky will look like based on the laws of physics. This may involve
lots of analytical work or computer modelling.
- Instrumentation, where tools are developed for use at the telescope.
- Many astronomers teach.
- Astronomers do public outreach.
What is going on with astronomy at NMSU?
- We have an astronomy department which consists of 8 faculty
members plus some research faculty. We offer a graduate PhD program,
but we don't offer an undergraduate major, partly because we feel that
astronomers should have a firm basis, e.g. an undergraduate degree, in
physics, and partly because we wouldn't get enough majors! We have
about 20 graduate students.
- NMSU operates a large observatory at
Apache Point, about 2 hours
of Las Cruces.
There are several telescopes at this site. The largest is
3.5m diameter,
and is shared with University of Washington, Univ. of Chicago, Princeton,
Washington State, and Johns Hopkins; NMSU gets about 15% of the time.
We also have a 1m telescope. There is also a
2.5m telescope
that is doing an a survey of all objects in a big piece of the sky; NMSU is a
partner in this project.
- Apache Point is ``next door'' to the National Solar Observatory
in Sunspot NM. There is a joint visitor's center in Sunspot.
- In the department at NMSU, we do a wide variety of research: solar system,
stars, galaxies, observation and theory.
- What do I do?
Overview of objects in the Universe
The Universe is LARGE, mostly EMPTY, DYNAMIC, and OLD
The Solar System
- The Solar System is the collection of objects which are associated with
the Sun by gravity.
- Contents of the Solar System and their properties:
-
Sun.
Biggest object in solar system by far and contains most of the mass
(> 99%). Shines with energy produced by nuclear reactions.
- Objects that go around the Sun
-
Planets.
Objects in roughly circular orbits, with roughly spherical
shapes (but a precise definition is very difficult!).
From a historical perspective, there are nine (but stay tuned!): in order from
closest mean distance from the Sun to furthest, they are
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune, and Pluto. All of the orbits are in approximately the same plane.
We can see planets because they reflect sunlight; in addition, as we will
see later, planets also glow because they are warmed by the Sun, but this
glow is mostly in a kind of light that our eyes don't see called
infrared light. Masses, sizes, and compositions
vary. There are two general classes of planets, the inner, rocky planets,
and the outer, gaseous planets. Pluto doesn't fit nicely in either
category.
-
Asteroids:
smaller rocky (usually) non-spherical bodies which orbit the Sun. Most
orbit between Mars and Jupiter in the asteroid belt.
Asteroids range in size from a few hundred miles down to very
small particles. The latter may also be called meteoroids.
When meteoroids collide with earth, they burn up in the Earth's
atmosphere, creating
meteors,
or shooting stars.
Occasionally, small pieces
make it down to the surface of the Earth; these are called meteorites.
-
Comets:
smaller icy bodies which orbit the Sun.
Comets are often in very elongated orbits around the Sun. They
spend most of the time far from the Sun, but when they come near the
Sun, material evaporates (sublimates) from their surface, and it is
pushed away from the comet by pressure coming from the Sun. This causes
comets to develop bright tails which can be seen from Earth when a comet
comes close to the Sun.
Appears that there are two families of comets: Kuiper belt and
Oort cloud.
- Recent discoveries of new ``planets'' and other objects:
2003UB13 discovery, size, and
orbit
(reference website)
Sedna
(reference website)
Kuiper belt objects and the status of Pluto: In past decade or so,
astronomers have been detected increasing numbers of objects that
at distances from the Sun equal to that of Neptune or even greater, and
these are known as Kuiper belt objects. Some of these objects are
relatively large, and this has raised the issue of whether they should
be called planets or not. In fact, many of these Kuiper belt objects have
properties similar to that of Pluto (ice-rock composition), so it is
possible that there is really a class of objects orbiting the
Sun at large distances, of which Pluto is the brightest. Perhaps it's
just a bit of semantics about what should be called a planet and what
a Kuiper belt object.
- What is a planet? Cultural definitions vs. ``scientific definitions''.
Does it matter?
- Moons or satellites. These orbit around planets.
Earth,
Mars,
Jupiter (4 big ones: Io, Europa, Ganymede, Callisto),
Saturn (1 big one: Titan)
(Titan atmosphere!),
Uranus,
Neptune (1 big one: Triton),
and Pluto all have moons. Some moons are spherical, others are not. Some
the moons of the outer planets are
nearly as big
as some of the inner planets! Our Moon is somewhat unusual in that is it rather
large compared to the size of the planet (Earth) around which it orbits; Pluto also
has a relatively large moon.
- Sizes in the solar system:
- The Sun is by far the largest object in the solar system. If the
Sun is represented by a circle 1 meter in diameter, than the Earth is
just 0.9 cm in diameter! Jupiter is the biggest planet, at 10 cm in our
model, and Pluto the smallest, at 1.6mm!
(Relative size of planets)).
- Even more striking, however, is the amount of empty space in the
Solar System. The objects in the Solar System are tiny compared
to the distances between them.
- Group exercise
In our model, the distance between the
Earth and the Sun is 107 m (distance to the sundial from our classroom),
and the distance to Pluto is 4.3km (the mall!).
(Sun is 1.4 million kilometers in diameter. The distance from the Sun
to Earth is about 150 million kilometers, i.e., about 100 times the
diameter of the Sun. The distance from the Sun to Pluto is about 6
billion kilometers, or 40 times the distance from the Sun to Earth.)
- Most of the Solar System is empty space!
- Motions in the Solar System
- Objects revolve around the Sun, because of gravity.
The shape of all orbits is a shape called an ellipse, which we'll
talk more about later; however, the shape of planetary orbits are
close to circular. All planets revolve in the same direction. All
of the planets revolve in approximately the same plane as well.
- Objects rotate around their own axes. Most rotate in the same
direction as the direction of revolution, but there are exceptions (Venus,
Uranus).
- The whole solar system moves through space.
- We've gone through a quick survey of what is in the Solar System.
However, remember that the science part of this consists of
other things as well:
- How did people come to figure out that this is what the Solar
System is like? How do we know where all of these objects are, how
big they are, and what they are made of?
- Why is stuff in the Solar System the way that it is? The reason
that the global properties of objects in the Solar
System are the way they are is likely related to the way
the Solar System formed. Whatever theories we have about how the Solar
System formed must explain some or all of the observed facts about the
Solar System: the revolution of the planets in the same direction and
plane, the difference between the inner and outer planets, why planets
are round, etc. In fact, there is a reasonbly well developed theory for
Solar System formation which we'll discuss later after we have some more
background in important physical processes. Outstanding questions: ...
The Milky Way Galaxy
- Our Sun is one of billions of stars which are held together by
gravity in what is known as the Milky Way galaxy. We now know that at
least some other stars have planets orbiting them, so it is likely
that there are many other ``solar systems'' out there.
- The distance between stars is much larger than the size of stars,
or even the size of the Solar System. On our scale model where the
Sun is 1m in diameter, the nearest star would be halfway around the
Earth! As in the Solar System, most of the galaxy is empty space.
- All stars are not the same.
- They come in different masses, with many more low mass stars
than high mass stars. Our Sun is of intermediate mass. Mass measures the
amount of matter in a star.
- Stars also come in different sizes, giving rise to the terminology
of giant stars and dwarf stars.
- Stars also come in different temperatures, which is related to
the color that they appear.
- Stars evolve: they are born, live their lives and die. For high
mass stars, lifetimes are millions of years, for low mass stars, billions of
years.
- Inside our galaxy, stars come as both individuals, in small groups
(binary stars, triple stars, etc.) and in clusters.
Star clusters come in two types,
open
and
globular
clusters.
Clusters have turned out to be very important for astronomers to understand
how stars work, because stars in a cluster all appear to be the same
age, but in a variety of masses. Binary stars are very important as they
are used to measure the masses of stars.
- Stars in our galaxy are not distributed uniformly.
- Most stars are found in a relatively thin disk of stars, which
has many more stars in its central regions than in its outer
regions. The Sun is located about 2/3 of the way out in this disk.
Looking through this disk from our location in the Milky Way is what
causes the
appearance of the Milky Way as a band around the sky.
- A minority of stars is found in a roughly spherical halo around the
disk. For some reason, globular clusters are found spread throughout the
halo, while open clusters are only found in the disk.
- Here is a diagram
of the shape of the Milky Way and of the location of objects within it.
Here is an actual
composite picture in infrared light
- There is stuff between stars which is called interstellar matter:
it is composed of
gas and dust
- There is a ongoing relation between the stars and the interstellar
matter in galaxies. New stars are
formed out of interstellar matter, and
old stars eject
much (or all) of their mass back into the interstellar
matter when they die.
- Inside our galaxy, both stars and the interstellar matter move. In
the disk, all stars orbit the center in the same direction in roughly
circular orbits, although it takes a few hundred million years for the
Sun to orbit the galaxy once. In the halo, stars do not all appear to
rotate in the same direction, and orbits can be highly elongated. As
with motions in the Solar System, these observations provide clues
about how the galaxy may have formed.
- As we discussed for the Solar System, some of the most interesting
stuff about this information we've given about the Milky Way galaxy is
an understanding of how we figure this stuff out, and how the Milky
Way got to be the way it is. Any theory of how our Galaxy formed
must account for the things we observe about it: it's shape, motions,
etc. Theories for galaxy formation are still in the process of being
developed. Outstanding questions:.....
Galaxies
- Our galaxy is just one of billions of different galaxies.
A galaxy is a collection of gas and stars held together by
gravity. All of the stars we see in the sky are in our own galaxy,
mostly relatively nearby to the Sun.
- Galaxies come in a variety of shapes. Galaxies are generally
classified in one of several categories:
- Spiral
galaxies have stars concentrated in a disk (which can either be seen face-on or
edge-on).
Some stars in the disk are concentrated in a spiral pattern, from which
this category gets its name. Spirals have stars as well as a significant
amount of interstellar matter.
- Elliptical
galaxies have stars which are found in an elliptical ball. Elliptical
galaxies can be nearly spherical or they can be
elongated.
Elliptical usually have less interstellar matter than spirals, with
very little dust.
- Irregular
galaxies don't fit into either category.
- The Milky Way galaxy is a spiral galaxy. Lab: how do we know this?
(Why does the Milky Way have its name?)
- Galaxies are very large. The distance between stars is much larger
than the size of individual stars.
- Motion inside galaxies:
- In spiral galaxies, both the stars and the gas move in roughly circular
orbits.
- In elliptical galaxies, the stars move, but in more irregularly shaped
orbits.
- Outstanding questions about galaxies....
The Universe of Galaxies
- Galaxies are not spread uniformly through space. They come in clumps
called galaxy groups and
galaxy clusters.
The largest clusters can have thousands of galaxies, which are held together
by gravity.
- On the largest scales, galaxies appear to be distributed in
filamentary-like structures. Many galaxy clusters and groups
are often found in proximity to each other along filaments, giving rise
to what are galaxy galaxy superclusters, but superclusters
are not isolated from each other in space in the way that galaxies are.
- We live in a small group of galaxies, composed of about 20 members,
called the Local Group. It is a few million light years across. There
are two big galaxies in this group (the Milky Way and
Andromeda), several
intermediate sized galaxies, and a bunch of small faint galaxies. The
closest galaxies to us are called the Magellanic Clouds (the
Large Magellanic Cloud
and the
Small Magellanic Cloud; they can
be easily seen, but only from the southern hemisphere.
- The nearest cluster of galaxies is the Virgo cluster, several tens of
millions of light years away.
- 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.
- 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.
- 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
- The 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.
- Within the context of the Big Bang theory, galaxies are thought
to arise from originally very small inhomogeneities that grow by
the force of gravity.
- This picture has received major support from observations in
the past decade by the COBE and WMAP satellities, which observe very
small ``lumpiness''
in the microwave background.
- What is the fate of the Universe?
- The expansion of the Universe is well understood in the context
of today's most sophisticated theory of gravity, which is called
general relativity. The equations of general relativity allow for
the possibility that the Universe will continually expand and also
for the possibility that it will eventually contract.
- The critical item that determines what will happen is the
density of matter in the Universe: if there is enough matter,
its gravity can evenutally cause the Universe to recontract
- Many recent observations, especially those made by the WMAP
satellite, strongly indicate that there is not enough matter
to cause the Universe to recollapse, in other words, that it
will continue to expand forever. These observations also
help to pinpoint the age of the Universe (how long ago the
Big Bang was) at 13.8 billion years.
- 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''
- What is the shape of the Universe?
- The amount of both matter and energy in the Universe also affects
the ``shape'' of the Universe.
- A useful
analogy
to describe the shape of the Universe can be found in two
dimensions when one considers the properties of a flat plane
vs. those of the surface of a sphere (or, separately, a saddle
shape). On these different surfaces, for example, the behavior
of parallel lines or the sum of the angles of a triangle
are different. If there is not enough matter and energy, the
Universe will be curved in an ``open'' sense, if there is just
the right amount, it will be flat, and if there is more than this,
it will be curved in a ``closed'' sense. The shape of the Universe
is related to its extent; an open universe has infinite size, while
a closed universe has a finite size.
- Recent observations suggest that the shape of the Universe is
``flat'', i.e. the analog of a flat plane in three dimensions. However,
the amount of matter in the Universe does not appear to be sufficient
to make it flat, hence the recent observations suggest that there is
a large component of
``dark energy''
in the Universe that in fact dominates the total matter-energy
density and makes the Universe flat. People are still trying
to figure out what this dark energy might actually be!
- 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
- 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 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
- 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.
- Distances is astronomy are difficult to measure, and for very
distant objects, as a result, they are not known with very high
precision. Determining distances to the most distant objects involves
several steps.
- In everyday experience, we can measure distances to many objects
directly. In astronomy, this is not practical for most objects, although
a form of it can be done within the solar system.
- For nearby astronomical objects, we can measure distances through
a technique known as parallax, which is the same technique used by
surveyors on Earth; it involves observing an object from two different
vantage points. We will talk more about this soon!
- For more distant objects, we look for objects which appear similar
to those at closer distances, and make the assumption that they have the
same intrinsic size or brightness as closer objects. The apparent
size or brightness of objects depends on the distance: more distant
objects appear smaller and fainter than identical nearby object.
- Measuring the apparent size of an object gives its distance
if we know its true size.
- Measuring the apparent brightness of an object gives its
distance if we know its true brightness.
- Some examples of objects used as standard candles, that
is, objects with constant intrinsic brightness are: a kind of variable
star called a Cepheid variable, and a particular kind of supernova
explosion.
- 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.
