The stars you see at night are our "nextdoor" neighbors. Literally.
Just as when someone takes a lit candle and walks away from you the candle ultimately becomes invisible getting dimmer and dimmer with increasing distance until it is so dim it can not be seen anymore. The same is true, as you would imagine, with stars.
Talking only about stars for the moment, the farthest we can see with the unaided eye is, generally, only in the 100's of light years away. Whether we can see a star with the eye depends on how truly bright the star is, some stars are bigger, and hence brighter, than the Sun, while others are smaller and dimmer. To see more one must use telescopes.
So we are, astronomically, extremely shortsighted. It's, astronomically, as if we could only see something if it was held up before our eyes less than a meter away.
Before modern astronomy and telescopes this was THE Universe. But, as you've no doubt heard, it has been determined that we live in a large collection of stars called the Milky Way galaxy, and that there are myriads of other galaxies strewn throughout the Universe.
Our literal "literal nextdoor" neighbor is Alpha Centauri which is the closest star to us (other than our own Sun of course) at 4.25 light years.
A light year is abbreviated "ly" and is equal to 9.46 × 1015 m.
We talked about Alpha Centauri in the last chapter and put its distance away from us into perspective by shrinking the Sun down until it had a 1 meter diameter. We shrink the rest of the Universe by the same scale and found Alpha Centauri would also be about 1 meter in diameter and more than 28,000 km away from our 1 meter Sun.
Before we begin to perspective-ize lets run the real life statistics for the Universe.
Our Milky Way is disk shaped and its diameter is about 100,000 ly across (9.46 × 1020 m). We live about 26,000 ly from the glactic center (2.45 × 1020 m).
So, how many stars does the Milky Way contain? We only have a best guess, since we can't see them all, and it is based on mass.
The Milky Way's mass can be inferred from the motions of neighboring galaxies and turns out to be in the range of 500 billion solar masses (abbreviated Msol). Remember the Sun has a mass of around 2 × 1030 kg.
Given that stars vary in size (from supergiant to normal Sun-like to red and brown dwarfs) a good estimate is maybe 100 to 300 billion? And we can see, with the unaided eye, only a few thousand of them.
The galaxy is dynamic of course and everthing is moving. Just as the planets orbit the Sun, the Sun orbits around the galactic center, taking about 250 million years to do so.
We mark birthdays using Earth's year which is simply one Earth trip around the Sun. A "galactic" year, then, is one Sun trip around the galactic center.
So, in very real sense, our Sun is only about 18-20 "years" old.
Evidence supports that our galaxy is spiral shaped like a pin-wheel. We can't see this directly for the same reason you can't see an entire forest while you are standing inside that forest. But you can see the forest in its entirety from a nearby mountain top or airplane.
Our literal "nextdoor" neighbor (major) galaxy is the Andromeda galaxy, also a spiral galaxy. There are two closer galaxies, visible by eye in the southern hemisphere, but they are irregularly shaped and not spiral galaxies, and are much smaller than either the Andromeda or Milky Way.
This picture of the Andromeda galaxy gives an idea of how our Milky Way looks:
Closest major galaxy to the Milky Way it also has a spiral structure and so this is somewhat how the Milky Way looks. Notice the other galaxy underneath it.
A larger version can be seen here (link will open in a new tab or window).
Photo courtesy of NASA.
The Andromeda galaxy is, in "real life", 2.5 million ly distant (2.36 × 1022 m). It has a mass of around 1.5 × 1012 Msol and so is about twice as big as the Milky Way containing, about, half a trillion stars.
An interesting aside fact here: This distance is the truly farthest you can see with just your eyes, no telescope needed.
On moon-less nights in a dark remote spot (cities are too bright) Andromeda appears as a smudge of light. Also interestingly, the smudge is, in apparent size, bigger than the full moon.
And here, just because they are so beautiful and so evocative, are a few more gratuitous pictures of galaxies:
Here are some pictures of other galaxies.
Just because why not.
Photos courtesy of NASA.
These pictures are sharp because these galaxies are "close" to us. To get a breath taking idea of just how many galaxies there are we need to look at the "far away" ones and we can do this by viewing the incredible "Hubble deep field" and "Hubble UltraDeep field" exposures:
Top: Hubble Deep field, Bottom: Hubble UltraDeep field.
Almost everything you see in these photos are distant galaxies (and hence they appear so small and indistinct).
The area of the sky the ultradeep picture was made was in the sword of the constellation Orion.
Photos courtesy of NASA.
The area of the sky the ultradeep picture shows is 2.4 × 2.4 arc minutes (the full Moon has a size of 34 arc minutes in diameter, an arc minute is one sixtieth degree). An area this size is about one thirteen millionth of the sky's area.
The best guess of the number of galaxies in the Universe, before this picture was taken, was around 200 billion. Afterwards the guess was upgraded to perhaps 2 trillion other galaxies, each with a myriad of stars and, in all likelyhood, many stars with more planets like Earth and Jupiter.
A beautiful graphic that shows the area of the UltraDeep exposure area.
A larger version can be seen here (link will open in a new tab or window).
Photo courtesy of Hubblesite.org (opens in new tab)
Now, you are probably also familiar with the notion of the "observable" Universe. No one knows how big the Universe is, we only know how far we can see out into it. The distance we can see is determined by the constancy of the speed of light. What is beyond this speed of light barrier? Well, the Universe. But not the "observable" Universe.
This observable Universal "horizon" distance in all directions is, in "real life", about 4.6 × 1010 ly (4.4 × 1026 m).
Finally, galaxies are organized, by gravitational attraction, into clusters which are clumped into superclusters, which then can be combined into a master superclusters. Clusters are usually gravitationally bound, but the superclusters may or may not be. Our "Local Group" supercluster (Andromeda, Milky Way, Andromeda, Large and Small Magellanic clouds, Triangulum, and some other small satellite galaxies, about 54 in all) belong to the Virgo supercluster which belongs to the Laniakea supercluster which holds about 100,000 nearby galaxies.
And superclusters align themselves into filamentous structures with HUGE void bubbles between them at the largest universal scales resembling the structure of a sea sponge.
So the Universe's structure is more complicated than one would imagine, but for this comparison we will assume the galaxies are spread out evenly.
Let's start shrinking the (observable) Universe.
We start by shrinking the Sun to the size of a ping pong ball (4 cm diameter), and the rest of the Universe by the same scale.
On this scale we find:
The Earth will be 0.33 mm in diameter and 4.28 m away from the ping pong Sun.
Jupiter will 4.1 mm in diameter and 22.5 m away.
Neptune, the "edge" of our Solar System, will be 1.4 mm in diameter and 129.24 m away from the "Sun".
At this shrinkage scale Alpha Centauri will be another ping pong ball about 1,150,000 m distant (1,150 km).
The center of the Milky Way would be 7.1 million kilometers distant, Andromeda would be 678 million kilometers distant, and the observable edge of the Universe would be 12.6 trillion kilometers away.
Hmmm. Looks like more shrinkage is in order here.
Let's shrink it so that our entire Solar System is the size of a ping pong ball. In this scale the Sun is 0.01 mm in diameter, smaller than a dust grain.
On this scale Alpha Centauri is another ping pong ball-sized star system and it would be 355 m away.
The galactic center would be 2,200 kilometers away, Andromeda 210,000 kilometers away, and the edge of the Universe would be 3.9 billion kilometers distant.
Hmmm. One last crank of the shrinking machine then:
We shrink it all down to the scale where the entire Milky Way is the size of a ping pong ball. Here both our solar system and Alpha Centauri's star system are separated by about a thousandth of a millimeter, and the Sun and Alpha Centauri sizes are on the order of atoms (more on those later in the book).
On this new scale the distance to the center of our galaxy is about 1 cm.
The distance to the Andromeda Galaxy is around 1 meter.
And the edge of the Universe is 18.6 kilometers away.
Picture it: you stand and stare into the distance and it is filled in all directions with 1 meter separated ping pong balls (some of the "galaxies" are actually smaller, say the size of a pea, and some are larger, say the size of a pool cue-ball).
You couldn't even see the edge of this shrunken Universe because the horizon distance for a person is only around 4 km and the "edge" here is almost 19 km away.
One final shrinkage:
Imagine you are at the center of a sphere with a radius of one kilometer (2 km diameter). Fill this volume with evenly spaced uncooked rice grains, each separated by about 5 cm.
This is our Universe. The Sun and Earth are well below the size of even protons (more on those later) at this scale, completely lost to perception in the vastness of the Universe.
Earth would be contained in one of the rice grains at the center of this bubble where you are standing, let's say the one in your belly button.
But this is not special. Remember, this sphere is based on a light "bubble", the observable Universe, as far as one can see in all directions from their current location.
Each rice grain is the center of its own light bubble. Take, for example, one of the rice grains about 900 meters away. It has it's own light bubble where Earth's rice grain is way over there, 900 m away, at that rice grain's edge.
And there you have it, a bird's eye view of the Universe.
While we have spaced out the rice grains evenly do remember that in actuality the rice grains are clumped a bit and arranged in strings and sheets like a sponge.
Resulting distances to astronomical markers.
Distance to Alpha Centauri
|Distance to Milky Way Center||Distance to Andromeda||
Distance to Universe Edge
|"Real Life"||4 x 1016 m||2.46 x 1020 m||2.36 x 1022 m||4.4 x 1026 m|
|Sun as PingPong ball||1,150 km||7.1million km||678 million km||12.6 trillion km|
|Milky Way as PingPong ball||--||1 cm||1 m||18.6 km|
|Milky Way as rice grain||--||--||5 cm||1 km|
In the next chapter we will go in the other direction and look at the small scale Universe.