Deep Sky

In the parlance of amateur astronomy, deep sky objects are any celestial sights other than the stars and solar system objects like planets, moons, asteroids and comets. Deep sky objects include other galaxies, like our neighbors Andromeda and the Magellanic Clouds, as well as star clusters and nebulae within our own Milky Way galaxy, like the Orion Nebula and the Beehive Cluster. This page, however, will include the stars in its scope providing information on the universe outside of our own solar system.

Star maps are always notoriously two-dimensional in nature, as technology to display 3D representations has not yet entered our homes. When viewing the images on this site, try to reconstruct the dimension of depth in your imagination.

 

Our Nearby Neighbors

Nearest Star Systems

This is one of my favorite pictures. This is our backyard in interstellar terms, a spherical radius of 12 light years. If we ever invent a faster-than-light propulsion system, we will be exploring these star systems first. Of particular interest is Epsilon Eridani since a planet has been discovered in this system based on observations where the star seems to "wobble" due to the gravitational pull of another body. The observations predict that the planet is a gas giant, like Jupiter, with an elliptical orbit. Where there is one planet, though, there may be others astronomers believe. In fact, computer modeling suggests the presence of another smaller planet, about a tenth of Jupiter's mass, closer to Epsilon Eridani than the larger planet. Here is a simulation of the orbit of the detected planets around Epsilon Eridani.

The Alpha Centauri system, which contains three stars, is also of great interest since not only is it the nearest star system to our Sun, but Alpha Centauri A is a yellow star of spectral type G2, exactly the same as the Sun. Alpha Centauri B, which is 23 astronomical units (AU) from Alpha Centauri A and is therefore gravitationally bound to it, is an orange star of spectral type K1. Alpha Centauri C, also called Proxima Centauri as it is the nearest star to the Sun, is a dim red dwarf star of spectral type M5, fainter and cooler than the Sun. It is only 4.22 light years from the Sun, where Alpha Centauri A and B are 4.35 light years away. Proxima Centauri is 13,000 AU from A and B, and it is not known if it is bound to them or if it will drift away from the system in some millions of years.

Alpha Centauri A is exciting to us because it passes all five tests for creating a promising place to find extraterrestrial life. (Alpha Centauri B passes all but one, but Proxima Centauri fails by only passing one test.) The tests are:


Unfortunately we do not yet know if the Alpha Centauri system contains planets. Here is an interesting simulation of the orbits of Alpha Centauri A and B, and the habitable zones which surround them.

You can explore some of the other nearby star systems on the superb Sol Station site. There is also a cool 3D map of nearby star systems on this site, which requires Java. Expand the scope of the simulation once using the question mark button and this will provide a spherical radius of approximately 13 light years, similar to the 2D star map above. A total of 1000 star systems are included in this simulation.

 

Our Galactic Neighborhood

Local Interstellar Cloud
The Local Bubble and the Galactic Neighborhood

 

The Constellations

 

The Milky Way

The Milky Way is our galaxy. Our Sun is moving through space in one of the Milky Way's sweeping spiral arms. The Milky Way is currently classified as large spiral galaxy like a giant pinwheel in space. More recent investigations have suggested that the Milky Way probably has a bar, or bar-like structure, in its central region. If so, this would change its classification to a barred spiral galaxy. You see, since we are sitting amongst the stars of the Milky Way it makes it difficult to see the true structure of our galaxy as we can with other distant galaxies.

On a dark, clear, and moonless night, well away from city lights, one can look up and see the Milky Way in all its glory. It appears as a band of thousands of pinprick stars, linked together by billowing wisps of stardust, that arcs across the whole of the night sky. Upon seeing the Milky Way like this it cannot fail to enrapture, as the mind attempts to construct its own enfeebled conception of the limitless reaches of space populated by billions and billions of stars, as Carl Sagan was fond of saying. In the summer the dense galactic center of the Milky Way can be seen in the direction of the constellation Sagittarius. From our perspective within the Milky Way, we see the cross section of our galaxy. The stars of the night sky that are not within the band of the Milky Way are our surrounding neighbors, in our local arm and the nearby spiral arms on this side of the Galaxy.

The Milky Way contains at least 200 billion stars. Most of these stars, as well as most of the dust and hydrogen gas, are contained within the galactic disk, the pinwheel shape of the Galaxy, both within and between the spiral arms. At the galactic center, the disk bulges out into what is called the nuclear bulge. The Milky Way also has a roughly spherical distribution of very diffuse, hot, highly-ionized gas called the halo. The halo contains globular clusters, described below, which are among the oldest stars in the galaxy. The disk, bulge, and halo are illustrated by this image.

The distance between our spiral arm and the next outer arm, the Perseus Arm, is about 6,500 light years. The galactic disk has a diameter of 100,000 light years, and it is about 27,700 light years from the Sun to the galactic center, which is believed to contain a black hole around which the Galaxy rotates. It takes the solar system about 226 million years to complete one rotation around the galactic center.

Details on the spiral arms can be found here.

 

The following images are from an excellent NASA Jet Propulsion Laboratory article called " Shedding Light on the Universe." The pictures show the Milky Way galaxy in different portions of the electromagnetic spectrum.


Milky Way Views

Gamma Rays

X-rays

Ultraviolet

Visible

Infrared

Radio

 

Deep Sky Objects

Andromeda Galaxy (M31)

Many of the spectacular images taken by the Hubble Telescope are of deep sky objects included in the Messier Catalog. This catalog was created by French astronomer Charles Messier during a 24 year period from 1758 to 1782. The celestial bodies included in the catalog are nebulae, star clusters and galaxies. Messier was a comet hunter, and these diffuse objects were difficult to tell apart from comets. He cataloged them so that they would not be mistaken as comets by other astronomers.

The Messier Objects can all be seen with amateur telescopes, and are therefore of great interest to amateur astronomers. There is a short period of time in late March when it is possible to see all the Messier Objects during the course of one night, and so amateur astronomers choose a new moon during this period and hold all-night Messier Marathons to attempt the task.

The Messier Catalog is an excellent web site providing information and images of the Messier Objects.

Two other well known astronomical catalogs containing deep sky objects are the New General Catalog (NGC) originally published in 1887 and Index Catalogs (IC1 and IC2), originally published in 1895 and 1907 respectively. You can browse these catalogs at the NGC/IC Project. Objects have been added to the catalogs since they were originally published.


Omega Centauri
Globular Cluster

Many of the stars in the Milky Way are gravitationally bound together into clusters. Concentrations of ten thousand to one million stars are called globular clusters. Most of the globular clusters populate the halo of the Galaxy, described above, though many are in the bulge. Studies have revealed that most globular clusters have highly eccentric elliptical orbits around the galactic center that take them as far as 100,000 light years into the halo. They do not participate in the Galaxy's disk rotation. As they follow their orbits over the course of billions of years, they are subject to a variety of effects which strip stars from the cluster. Therefore, the existing globular clusters are just the survivors of a much larger population of clusters, which have since been disrupted and spread their stars throughout the halo. This process of destruction is still happening, and it is estimated that about half of the current Milky Way globulars will no longer exist in 10 billion years time. Here is an article about studies of Palomar 5, a globular cluster being shredded by the Milky Way.


Butterfly Cluster (M6)
Open Cluster

Open clusters are different from globulars in both appearance and nature.
"They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.

Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years." -- SEDS


Orion Nebula (M42 and M43)
Diffuse Nebula

Nebulae are clouds of dust and gas in space. There are four types of nebulae. Diffuse nebulae are conglomerations of dust and gas. If they are massive enough, they are frequently a place for new star formation, like the well known Orion Nebula (M42). The star-forming portion of the Orion Nebula, which is called the Trapezium cluster is visible even with an amateur telescope. If the stars inside the nebula are hot enough, they can excite the gases of the nebula (mostly hydrogen) so much that it shines. This is called an emission nebula. If the stars are not hot enough, their light can still be reflected by the gases of the nebula appearing white or bluish. This is called a reflection nebula.


Horsehead Nebula
Dark Nebula

Dark nebulae are also clouds of dust and gas, but unlike diffuse nebulae they are not illuminated by embedded or nearby stars. They can only be seen when they absorb the light from a brighter object behind them, hence they are also called absorption nebulae. The Horsehead Nebula (Barnard 33) is a well known example of a dark nebula.

Diffuse and dark nebula are often called star-forming or pre-stellar nebulae as they are composed mostly of clouds of interstellar matter that has not yet formed into stars. Typically the mass of this matter is that of several hundred to several thousand stars. The next two types are nebulae are called post-stellar nebulae and are related to one specific star in the later stages of stellar evolution.


Ring Nebula (M57)
Planetary Nebula

Planetary nebulae are the first type of post-stellar nebulae. William Herschel coined the name "planetary nebula" in 1785 because to him they closely resembled the planet Uranus, which he had just discovered. In fact, a planetary nebula is a star which has used up all its nuclear fuel and has finally ejected a significant portion of its mass in a gaseous shell, which then emits light due to excitation by its extremely hot central star. The central star is what was previously the core of the original star. Planetary nebulae quickly expand and then fade as the gaseous shell diffuses into the interstellar surroundings.


Crab Nebula (M1)
Supernova Remnant

Supernova remnants are the last type of nebulae. A supernova remnant is a star after its nuclear life cycle has ended in a violent death. Very massive stars, when they have reached the end of their life cycle, can finally explode in a violent detonation called a supernova. The explosion causes most of the star's stellar matter to be ejected into space in an expanding and slowly fading gaseous shell, which is a visible nebula.

The Crab Nebula pictured above is the result of a supernova that was recorded by Chinese and Europeans in 1054 A.D. The supernova was so bright it could be seen during the day!

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Please contact Adam if you have questions or comments about this page. Research and image sources are provided when possible.

Sources:

Images:
Images are shown here for noncommercial educational purposes.
Images are from the SEDS Messier Database site unless otherwise noted.
The star map of our nearby neighbors on this page was created by Richard Powell and is on his site of star maps.
The source of the relative sizes of the Alpha Centauri stars image is here.