The Earth's atmosphere is the bane of astronomers. The idea of sending a telescope into space to avoid light, dust, and clouds was first proposed long before the first satellites were launched, long before anyone even dreamt of sending astronauts to space. A space telescope avoids frustrating problems such as cloudy and misty observing nights, the twinkling of stars even on clear nights and absorption of the ultraviolet and infrared parts of the spectrum. The Hubble Space Telescope is a joint ESA/NASA project and was launched in 1990 by the Space Shuttle mission STS-31 into a low-Earth orbit 569 km above the ground. During its lifetime Hubble has become one of the most important science projects ever. Hubble's orbit above the Earth's distorting atmosphere allows astronomers to make the very high resolution observations that are essential to opening new discovery windows onto planets, stars and galaxies. Hubble was designed as a high standard flagship mission and has paved the way for other space-based observatories. Notably it can access the otherwise invisible ultraviolet part of the spectrum, and also has access to areas of the infrared not visible from the ground. It has had an incredible 30+ year in orbit.
As of spring 2018, the Hubble Space Telescope has made over 1.5 million observations of more than 43,500 celestial objects. The 30+ years worth of observations has produced more than 153 terabytes of data and, the orbiting observatory generates more than 80 gigabytes of data each month. Astronomers using Hubble data have published more than 15,500 scientific papers, making it one of the most productive scientific instruments ever built my man. One of the main scientific justifications for building Hubble was to measure the size and age of the Universe and test theories about its origin. Hubble is able to offer sharper visual light images than any telescope currently in operation on the ground. This means that it is well suited to studying study stars in nearby galaxies. While most galaxies cannot be resolved into the individual stars they are made up of, the Milky Way’s neighbours, including the Magellanic Clouds and the Andromeda Galaxy (M 31, the closest spiral galaxy to the Milky Way) can now be broken down into hundreds of millions of individual points of light, and their stars studied individually. Hubble has even been able to resolve star clusters in the Andromeda Galaxy, despite it being around 2 million light years away. It has also discovered that the stars in the halo (the sparse region around a galaxy’s disc) of M31 are significantly younger than those of the Milky Way. Hubble's ability to study stars in nearby galaxies is superior to any ground-based telescopes, and its ability to observe ultraviolet light, important for studying young stars, is unique.
To astronomers and laymen alike the topic of star formation has always been a particularly appealing one. The reason being that important clues about our genesis lie hidden behind the veil of the dusty, and often very beautiful, star forming molecular clouds. Our Earth and the Solar System were born 4.6 billion years ago and our knowledge of the event is sparse. Astronomers turn their eyes to the birth of other stars and stellar systems in neighbouring stellar maternity wards and use these as a time machine to see a replay of the events that created our own Solar System. The chemical composition of the Universe and the physical nature of its constituent matter are topics that have occupied scientists for centuries. From its privileged position above the Earth’s atmosphere Hubble has been able to contribute significantly to this area of research. All over the Universe stars work as giant reprocessing plants taking light chemical elements and transforming them into heavier ones. The original, so-called primordial, composition of the Universe is studied in such fine detail because it is one of the keys to our understanding of processes in the very early Universe.
Hubble's sensitivity and high resolution allow it to see faint and distant gravitational lenses that cannot be detected with ground-based telescopes whose images are blurred by the Earth's atmosphere. The gravitational lensing results in multiple images of the original galaxy each with a characteristically distorted banana-like shape or even into rings. Hubble was the first telescope to resolve details within these multiple banana-shaped arcs. Its sharp vision can reveal the shape and internal structure of the lensed background galaxies directly and in this way one can easily match the different arcs coming from the same background object, be it a galaxy or even a supernova, by eye. As gravitational lenses function as magnification glasses it is possible to use them to study distant galaxies from the early Universe, which otherwise would be impossible to see.
Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalog), the IC (Index Catalog), the ARP (Atlas of Peculiar Galaxies), the CGCG (Catalog of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalog of Galaxies) and UGC (Uppsala General Catalog of Galaxies). All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. This website uses the NGC (New General Catalogue) and will cross reference them with their Messier Catalog number in the list at the bottom of this page.
A galaxy is a gravitationally bound system consisting of stars, stellar remnants, interstellar gas and dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with just a few thousand stars to giants with one hundred trillion stars, each orbiting their galaxy's own center of mass. Galaxies have historically been categorized according to their visual morphology, including elliptical, spiral, irregular, and starburst. Many galaxies are believed to have black holes at their center. The Milky Way's central black hole, known as Sagittarius A, has a mass four million times that of our Sun.
It's hard to imagine the observable universe containing hundreds of billions of galaxies. Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas with an average density less than one atom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids. We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses; our Sun is one of several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant. While the nearest giant neighbor, the Andromeda Galaxy (M31), also a spiral, is about 2-3 million light years distant.
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A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Most nebulae are of vast size, reaching sizes of even hundreds of light years in diameter. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created in an Earthen environment - a nebular cloud the size of the Earth would weigh only a few kilograms. Nebulae are often star-forming regions. In these regions the formations of gas, dust, and other materials clump together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. Objects named nebulae belong to four major groups: H II regions, large diffuse nebulae containing ionized hydrogen, planetary nebulae supernova remnant, and dark nebula. Not all cloud-like structures are named nebulae; Herbig–Haro objects are an example.
Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula. Some nebulae are formed as the result of supernova explosions, the death throws of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the Crab Nebula, in Taurus as seen below. Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off.
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Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group. Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster.
Stellar clusters are important in many areas of astronomy because the stars were all born at roughly the same time. The different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters. Globular clusters are roughly spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30 light years across. Open clusters are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as M67, the closest open cluster. Embedded clusters are stellar clusters of stars that are partially or fully encased in a nebula.
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The Milky Way Galaxy is our home galaxy. It's a large spiral system consisting of several hundred billion stars, one of which is the Sun. It takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth. Although Earth lies well within the Milky Way Galaxy, astronomers do not have as complete an understanding of its nature as they do of some external star systems. A thick layer of interstellar dust obscures much of the Galaxy from scrutiny by optical telescopes, and astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes, which can detect the forms of radiation that penetrate the obscuring matter.
Astronomers have learned that it’s a large spiral galaxy, similar to many others, but also different in ways that reflect its unique history. Living inside the Milky Way gives us a close-up view of its structure and contents, which we can’t do for other galaxies. At the same time, this perspective makes it difficult for astronomers to obtain a complete picture of galactic structure. Modern research on the Milky Way refines our understanding of how the galaxy formed and what continues to shape our galactic home. On a dark clear night you will see a milky band of light stretching across the sky. This band is the disk of a galaxy, our galaxy and is made of millions of stars along with a lot of gas and dust.
Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars. Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. The idea that each star is a sun, many with their own solar systems, is a powerful reminder of the immense scale of the cosmos. However, the distances to stars in our galaxy are tiny in comparison to distances to other galaxies.
The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars (more recent estimates have given numbers around 400 billion) and their planets, and thousands of clusters and nebulae, including at least almost all objects of Messier's catalog which are not all galaxies. As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Our galaxy has both a pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component. The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. In the infrared light, the structure of the Milky Way can be better investigated, as the obscurring dust clouds are of better transparency for long wavelength IR than for the visible light.
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