The Hubble Deep field image shows us that there are very many galaxies, some similar to our own Milky Way galaxy. Since we reside within the Milky Way, we cannot yet see how it appears from a distance. Perhaps some day we will venture outside our galaxy, but for now, our earliest probes are barely leaving our solar system. Even so, astronomers have done a lot of work to understand our galaxy and how it is laid out in space. Estimates vary between 100 billion and 200 billion as to how many stars are in the Milky Way.
The Milky Way is some 100,000 light years across. It is extremely hard for us to grasp distances that large. One way to gain an understanding of this vast distance is to compare it to sizes in our everyday lives, in decreasing scales by powers of ten. The link below takes you to one such journey, beginning with a representation of the Milky Way galaxy, decreasing to our everyday world, and continuing on down to subatomic sizes.
The above image shows the relative sizes of the planets in our solar system. The bottom row is Earth, Venus, Mars, Mercury and Pluto (minor planet).
It is notable that Jupiter is by far the largest planet in our Solar system. The gas giants Jupiter and Saturn and the ice giants Uranus and Neptune are considerably larger than the terrestrial planets.
Of the terrestrial planets, Earth and Venus are about the same size, while Mars is considerably smaller. Mercury is quite a bit smaller than Mars, and Pluto is relatively tiny.
This image shows how big the sun is compared to the planets in our solar system. While Jupiter is very large compared to earth, it is small compared to the Sun, like a golf ball next to a beach ball.
In comparison to some other stars, our sun is actually a rather small star. Antares and Betergeuse are supergiant stars, our sun is only one pixel in this image. If Antares was a large beach ball, the Sun would be a grain of sand.
constellations in the night sky image source
Since ancient times, people have noticed patterns in the stars of the night sky. Stories were built up around the groupings, or constellations that ancient folks identified. These constellations became a backdrop for the stories and lore that was passed down through the generations.
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The Celestial sphere
Ancient people mapped the heavens and thought of the stars as residing on the surface of a great sphere surrounding the world, called the celestial sphere. We now know that stars do not all lie at the same distance from us, but the idea of the celestial sphere is still used, since it is a convenient way to describe the locations of the stars in the night sky, as viewed from earth.
The above image is a time-lapse photograph that shows the paths stars take over the course of a few hours. The constellations appear to move across the sky at night. Of course, it is really the result of earth’s rotation that we are noticing. Stars do move, but they are so far away that we cannot discern their motion over the course of one night. The star that happens to lie directly over the north pole seems to stand still. We call this the North Star, or Polaris. You can locate Polaris by finding the Big Dipper, and tracing a line from the front two stars to the tail of the Little Dipper. If you watch the Big Dipper over the course of a night, you will see that it rotates around Polaris. We call stars that do this “circumpolar stars.” If you look away from the north, you will see that the stars still seem to have a curved progression across the sky. There is currently no star that sits just above the south pole, but there are circumpolar stars in the southern hemisphere.
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Over the course of a year, earth takes a lap around the sun. At any given time of year, some stars will not be visible to us, because the lie behind the sun or are close enough to it that they are only “out” in the day. The constellations that lie along the path that the sun appears to take in the sky are known as the zodiac stars. Ancient people such as the builders of Stonehenge and other early “observatories” were very adept at keeping track of the apparent motions of constellations, and developed calendars built on the knowledge that they gained by watching the heavens.
Measuring distances in astronomy is a very important aspect of understanding the interactions between bodies in the universe and the dynamics of the universe as a whole. Since there is such a wide range of distances to consider, we need several methods to determine distances between objects. We will find that calculating the distance to "nearby" objects like the nearest requires a very different method than is necessary for calculating distances to galaxies.
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To get a better idea of how this works, check out this Stellar Parallax simulation.
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This image shows the distances to a few nearby stars. The closest star is Alpha Centauri, 4.367 light years away, or about 1.3 parsecs. As this diagram shows, there are about two dozen stars within 3 parsecs of the Sun.
We cannot use stellar parallax to measure distances to stars that are considerably farther away (over a hundred parsecs), since their apparent motion across the sky is so small that their parallax angles are not measurable.
For more information, see Las Cumbres Observatory
To measure distances to more distant stars, we use a method called "spectroscopic parallax. Basically, the idea here is that stars spend most of their lives shining at a certain intrinsic brightness, or luminosity, that depends on their mass and surface temperature. We compare this intrinsic brightness to how bright they appear to us to estimate the distance. This method is good to about 10,000 parsecs away, and is covered in more detail in PH 206. If you are interested, please see section 17.6 of your text. We will continue to study methods of measuring distances, increasing our range step by step until we reach galactic scale and large scale of the universe.
Notice that it is not possible to view the relative size of the sun and Earth and also their distance apart in this small diagram. There is just too big of a difference in the relative length scales. What if we had more room? Let's look at how we could build a scale model of our solar system. We need to start by choosing one object's size. Let's assume that in our model, the sun is the size of a bowling ball. Comparing the size of a bowling ball will allow us to calculate the other relative distances in our scale model. We also need to know the real distances involved.
Here, d is the diameter of "Earth" in our scale model.
The ratio of the ball to the sun is the same as the ratio of the size of earth in our scale model to the real size of the earth. We can set these two ratios equal to each other to find the diameter of "Earth" in our scale model.
Solving for d, we see that in our model, Earth is the size of a small bead.
Let's find out how far our model Earth is from our model Sun.
We will let "D" be the distance between the Earth and Sun in our model.
The distance 1 AU (astronomical unit) is the distance between the Earth and Sun.
We can use the same ratio to calculate how far apart the bead would be from the bowling ball in our model. Here, we can see that the distance in our model would be 24 meters.
Now, if the true distance to the nearest star, Alpha Centauri, is 4.0 x 1016 m, how far away would it be in our model?
© Kathryn Hadley PhD 2020
We will begin studying the universe by examining our place in the cosmos. Here, we will take a look at how we fit in as far as size and distance. We will also take a look at how the relative motion of Earth affects our perspective of the universe around us. We will also consider some of the underlying physics that we will need to understand the dynamics of Earth and the universe as a whole.