The Big Picture covers a patch of the sky 15.2 degrees long and 2 degrees wide, which is only about 1/1357th of the entire celestial sphere.  In depth, it ranges from the Solar System (distances of hundreds of millions of miles, or light minutes) to very distant quasars (distances up to 12 billion light years away).  (A light year is the distance light travels in a year at the speed of roughly 186,000 miles per second - or nearly 6 trillion miles long.)

Where is it on the sky? If you go out early on a clear summer evening, high above the southern horizon there will be the constellation of Virgo (the Maiden).  The Big Picture covers a small part of that constellation. It is looking almost straight up from the plane of our Galaxy, the Milky Way.  Thus there are (by choice and design) a few bright stars in it.  It is the part of the sky which the astronomer Edwin Hubble called "the realm of the nebulae", which is how other galaxies were called at that time, in the 1920s.  Indeed, the picture covers the core of the Virgo Cluster of galaxies, the nearest one to the Earth, some 60 million light years away. The largest galaxies you see in the picture belong to the Virgo Cluster.

Here are some kinds of astronomical objects you can see in the picture:

Stars in our Galaxy: there are nearly half a million of them in this picture.  They range in distance typically from hundreds to thousands of light years, and generally the closer stars appear brighter (bigger on the picture), but there is also a large spread in how luminous they really are.  You can spot them as relatively sharp circular dots on the image, larger being brighter.  If the picture was taken from the space, they would be much sharper, but the turbulence in the Earth's atmosphere blurs them. This blurring is what you can observe as twinkling of the stars.

Our Sun is a fairly typical star, and it appears as big, bright, and hot because it is by far the nearest one to us; the next nearest star is about 300,000 times further away, but most other stars are tens of millions to a billion times further away - thus they appear as faint dots,
compared to the daytime Sun.  Stars range in age typically from millions to billions of years; our Sun (and the rest of the Solar System) is about 4.5 billion years old.  Stars are big and massive: our Sun is roughly a hundred times larger than the planet Earth, and it is roughly 300,000 times more massive.

What gives stars their colors? It is their surface temperature. Hotter stars appear bluer, and cooler stars appear redder.  Hot, blue stars are generally much more massive and more luminous than our Sun, but they also live shorter; but there are also small hot stars (roughly the
size of the planet Earth), called the white dwarfs.  Red stars also come as dwarfs or giants; they tend to be longer lived, as they burn their fuel more slowly.

Why do stars shine? They are gigantic thermonuclear furnaces.  In their cores, there is a constant, gigantic thermonuclear explosion going on - but the star's huge gravity holds it together.  Stars burn the lightest element, hydrogen, into the heavier elements, especially helium. That process releases huge amounts of energy, which eventually escapes
from the stars as the light we see.

Galaxies are gigantic groupings of stars - typically a hundred billion of them.  The used to be called "island universes".  Our own Galaxy, the Milky Way, contains about 200 billion stars, and it is a fairly typical galaxy.  Its nearest large companion is the Andromeda Galaxy, also called M31, but there are also tens of much smaller, dwarf galaxies circling around these two giants.  Galaxies have sizes of thousands to tens of thousands of light years across, and could be a trillion times more massive than our Sun.  There are about a 100 billion galaxies in the observable universe. Large galaxies come in two main types, spirals, like our own Milky Way, and ellipticals.  You can see many examples of both kinds in the Big Picture.

Spirals are flat and disk-like, and spin around.  They often have a little elliptical galaxy in the middle, called the bulge.  They are constantly making new stars in their spinning disks, which gives them bluer colors, since the light from younger stellar populations is dominated by the hot, massive stars which don't live very long.  In addition to stars, these galaxies also contain lots of gas and dust clouds (from which young stars are born), and a lot of the mysterious dark matter.

Ellipticals are more spherical, oval, or football-shaped.  They don't spin very much, but stars within them swarm in random motions.  They are made of older stars, and most of the light comes from the so called red giant stars.  There is little or no new star formation going on - most of it expired bilions of years ago.  They also contain some very hot gas (temperatures of millions of degrees), and the ubiquitous dark matter.  It is generally believed that elliptical galaxies are made from mergers of spirals or smaller galactic pieces.

Dwarf galaxies are hundreds or thousands of times fainter than the larger galaxies we just described, but they are much more numerous.  They are hard to spot, because they are both faint, and diffuse (very low in contrast).  They came in a variety of forms, some with gas and making stars, some not.  But they always have a lot of the dark matter.

What is this dark matter anyway?  No one knows, but it dominates the mass budget of the universe. There is about 5 times as much of it as there is of the known kinds of matter, made out of atoms.  We know this because we can measure its gravity, even if we cannot see it.  It was discovered in the 1930's by the Caltech scientist Fritz Zwicky, but its importance has been realized only for the past 30 years or so. Explaining the nature of the dark matter is one of the outstanding problems of physical sciences.

Galaxies like company. Our Galaxy, the Andromeda, and tens of nearby dwarfs, make up the Local Group of galaxies.  This group is on the outskirts of the Local Supercluster of galaxies, in the center of which is the Virgo Custer of galaxies, at which you are looking in the Big Picture. Clusters of galaxies contain hundreds or thousands of galaxies, along with copious amounts of dark matter.  Virgo is a relatively poor cluster, but it is the nearest one to us.  Around 10 percent of all galaxies live in clusters, and most of the rest in galaxy groups.  A
typical cluster is a few million light years across.

Clusters and groups sometimes make larger structures, superclusters. They are the largest structures known to exist in the universe, and are hundreds of millions of light years across.  All these groupings form what is known as the large scale structure in the universe.  It forms by gravitational attraction of mass, as galaxies and their groupings fall towards each other and larger mass concentrations. Thus, the structure in the universe is constantly evolving.

Galaxies evolve. They are made of stars, which evolve themselves, and the rate by which they make stars also changes in time.  Moreover, galaxies frequently run into each other, and sometimes merge. There are some examples of this process seen in the Big Picture.

Most or all large galaxies also contain massive black holes in their centers.  Our own Galaxy contains a black hole 2 or 3 million times the mass of our Sun.  But a giant elliptical galaxy M87 seen in the Big Picture contains a black hole hundreds of millions times more massive than the Sun. All that mass is squeezed into a volume roughly of the size of the Earth's orbit, generating a very strong gravitational field.

As matter falls into the black holes, it speeds up enormously, collides, and releases huge amounts of energy. This creates very luminous objects called quasars.  There are nearly a thousand of them seen in the Big Picture. They can outshine their entire host galaxy, sometimes by a factor of a thousand. Yet, in a picture they look just like stars - but the appearances are deceiving: they are simply very far away, often billions of light years.

In some cases jets of mass and energy are emitted from these active galactic nuclei.  A well known example is seen in the galaxy M87.

On the other end of the scales, there are small bodies in the Solar System, which can be simply rocks - asteroids - or dirty snowballs - comets. There are many millions of them circling the Sun. There are about a thousand of asteroids captured in the Big Picture - sometimes seen as series of equally spaced "stars", if they are captured in multiple scans as they moved relative to the background starry sky. There is also at least one comet (called P/Tsuchinshan), also captured multiple times, in different filters; see if you can spot it.

These cosmic rocks sometimes hit the Earth. The smaller ones are called meteors, and you can observe them as "shooting stars" as they burn in the Earth's atmosphere.  The big ones can cause planetary scale mass extinctions; one of them did the dinosaurs in, roughly at the time when the light we now see from the Virgo Cluster galaxies started on its way, about 60 million years ago.