PART 2 - OVERVIEW OF THE UNIVERSE

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 %). It shines with energy produced by nuclear reactions.

  • Many objects that go around the Sun
    • Traditional classification into: planets, asteroids, comets, moons. Planets are biggest objects that orbit the Sun in roughly circular orbits, asteroids are small rocky objects that orbit the Sun, comets are combinations of rock and ice that orbit the Sun often in elongated orbits, and moons orbit around planets.

    • We can see the objects that orbit the Sun because they reflect sunlight. The brightness of an object depends on its distance from the Sun, its distance from Earth, and how big it is. In addition, as we will see later, planets also glow because they are warmed by the Sun (and a few have an internal heat source as well), but this glow is mostly in a kind of light that our eyes don't see called infrared light.

    • While some solar system objects have been known for a long time, many have been discovered recently, and more are continually being discovered!

      $\bullet$ How do you discover a new Solar System object?

    • Some current maps from the Minor Planet Center . Most of the objects are very small. The amount of space between objects is much larger than the objects themselves!

    • Planets are the brightest orbiting objects in the solar system, with roughly spherical shapes. From a historical perspective, there are nine planets: in order from closest mean distance from the Sun to furthest, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

      $\bullet$ Orbits of planets are roughly, but not exactly circular. They also generally orbit the Sun in the same plane.

      $\bullet$ By orbits, however, Pluto is a bit different, both in its shape and its inclination (tilt of orbit relative to plane of solar system).

      $\bullet$ See an orbit animation to see these points. Considering the planets, what is the shape of the Solar System?

    • With the new discoveries, it appears that Pluto is one of the larger of a group of objects that orbit out past the orbit of Neptune. All of this has made the whole old style of classification less clear. Perhaps it is better to talk about several groups of minor planets: main belt asteroids, Plutinos, etc.

      $\bullet$ Recently, it's become popular to call the largest objects (photos) in the solar system planets or dwarf planets, depending on whether they are much larger than other objects orbiting near them.

      $\bullet$ Note the distinction between masses and sizes of planets. An introduction to graphs: the range of masses and sizes among planets.

      $\bullet$ Is there an easy way to tell if planets are all made of the same stuff? Group question: masses and volumes of objects

      $\bullet$ Group question: are all the planets made of the same material?

      $\bullet$ Some idea about the compositions of the planets can be determined by calculating their average density. The average density is determined by dividing the mass of the planet by its volume. It's only a rough estimator of composition, but it indicates at least two different classes of planets: rocky planets and gaseous planets.

    • 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, and they have solid surfaces. 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.

    • Group question: utility of classification

      $\bullet$ Often it's useful to classify just to be able to digest the information.

      $\bullet$ Sometimes what seems like a sensible classification turns out not to be all that relevant physically, and classification schemes can change as one comes to learn more about objects

      $\bullet$ In general, one wants a classification scheme that gives real information about the origin of the differences between objects. For example, in the Solar System, objects that form in different ways would reasonable be placed in different categories.

  • Sizes in the solar system:
    • The Sun is by far the largest object in the solar system.

    • Group exercise: relative sizes of Solar System objects

      $\bullet$ image showing 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: distances between objects

    • 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.

      $\bullet$ There is very tight relation between the orbital periods, orbital speeds, and the average distance of the object from the Sun. Anyone seeing this relation might reasonably wonder why!

    • Objects move around the Sun, and since they're in a variety of different orbits, they sometimes crash into one another. This is especially true for the smallest objects because there are lots of them, and some are in more elliptical orbits. Collisions between big objects are pretty rare now because although there's a lot of objects, there's a lot of space between them! But it still happens sometimes. In the past, it probably happened more, and has been an important process in shaping the surfaces of solar system objects.

      $\bullet$ When small objects 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.

      $\bullet$ When big objects collide, the impact can be significant!

      $\bullet$ 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.

    • 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.

  • Group question: lessons learned and open questions
  • 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 models/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. But there are still many 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. However, stars are so far away that we can't directly see how large they are, but instead have to infer their size from understanding the light they emit; here is a link to a site that shows some schematics of relative sizes of stars (but these are not real pictures of stars!).
  • Stars also come in different temperatures, which is related to the color that they appear.

• 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 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.

• 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.

• What is the shape of our Galaxy?
  • Some thought experiments

    $\bullet$ Group question: galaxy shapes

    $\bullet$ Group question: galaxy shapes

    $\bullet$ Group question: galaxy shapes

    $\bullet$ Group question: shape of the Galaxy

  • 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. However, we don't see far into the disk because of the dust that is there.

  • 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. We see many more globular clusters in one direction than in the other, which is one of the observations that indicates that we don't live in the center of the Galaxy.

  • An interactive map of our galaxy and its location among other galaxies

  • 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, which shows the shape of the Galaxy much better than looking in visible light because infrared light penetrates through dust much better. Here are a series of composite pictures taken with lots of different types of light.

• Group question: scale of the Milky Way

• 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 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.

• 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.

• 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.

• 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. The Milky Way galaxy is a spiral galaxy.

  • 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.

• Galaxies also come in a large range of sizes and masses. The stars are not all the same from one galaxy to another: some galaxies have predominantly old stars, while other galaxies have range of ages.

• Galaxies are very large. The distance between stars is much larger than the size of individual stars.

• Motion inside galaxies:
  • In the disks of spiral galaxies, both the stars and the gas move in roughly circular orbits, in the same direction. (In the sparsely populated halos, stars move in more elongated orbits, and in different directions.
  • In elliptical galaxies, the stars move, but in more irregularly shaped orbits (similar to the halo stars to spiral galaxies).

• Outstanding questions about galaxies: why are there several different types of galaxies? Is there something different about how different galaxies form? Or something different about what happens to galaxies after they've formed?

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 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 - how do we know how far away things are?

• 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.
  • Group question: 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 $$\propto$$ $${\mathrm{true size} \over \mathrm{distance}}$$
      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!

  • For more distant objects, we look for objects which appear similar to those at closer distances under the assumption (which can be tested) that they have the same intrinsic size or brightness as closer objects. The apparent brightness of objects depends on the distance: more distant objects appear smaller and fainter than identical nearby object.
    • Group question: measuring distances with brightnesses

    • 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 $$\propto$$ $${\mathrm{true brightness} \over \mathrm{distance}^2}$$

    • 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 know what the true brightness is by studying the type of light that stars produce, and recognizing from measuring distances to nearby stars that the brightness are corrlated with the type of light. (Lightbulb demo: how brightness changes with distance, how you can have lightbulbs of different intrinsic brightness, and how you might recognize the difference, e.g. lightbulbs with different colors).

    • 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
  • asteroids
  • comets
  • dwarf planets
  • inner planets
  • outer planets
  • Sun: light minutes away
  • solar system: light-hours across
  • 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) movie, Powers of 10, which is available on Youtube (link)

• 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.