"The nitrogen in our DNA, the calcium in our teeth, the iron in our blood...were made in the interiors of collapsing stars. We are made of starstuff."
Stellar astronomy is a wide ranging discipline of astrophysics that centers on the characteristics of and evolution of stars. Far from being mundane and similar in characteristics, stars vary greatly in their size, chemistry, age and evolutionary progression that demands a much closer look than typical high school textbooks provide. In addition, an understanding of the nature of stars and the dynamic processes that create them lead us to understand our own creation and how the sun will change with age along with its ultimate end several billions of year into the future.
This unit includes a myriad of reading sections and practical lab investigations designed around this basic notion of astronomical study. This unit also includes additional material designed around helping students prepare for outdoor star gazing activities with or without the use of sophisticated telescopes.
Lets assume its a clear, cool night and, while outside, you notice
The open star cluster NGC 602 and nebula N90 located on the periphery of the Small Magellanic Cloud. These young, very hot blue stars have blasted the surrounding gas clouds of the nebula away from them and spurred on further star development along the edges of the nebula. Image: STSci
a bright object in the southern sky that you have not noticed before. You decide to text a friend who knows a lot about astronomy but lives on the West Coast. How would you describe exactly where this object is in the sky?
If you are really thinking about a solution, you should come to the quick realization that where the object appears to you is not going to be the same location it will appear to your friend who is 3000 miles away. Not only is the time on the West Coast several hours off, your friend’s latitude may also be different. So, how would you accurately describe to an observer somewhere else on Earth the location of an object in the sky? There are two main ways to do this.
The Earth Coordinate (Horizonal) System
More than likely, the first method you thought of for identifying the location of an object in the sky was what is known as the Earth Coordinate (Horizonal) System. This system relies on two measurements to triangulate an object’s location. First, the viewer must consider what direction he/she is facing on a compass from 0-359 degrees. This direction is known as an azimuth and tells another observer where to face along the horizon. The second measurement that needs to be taken represents how high in the sky the object is (its altitude) from 0 degrees on the horizon to 90 degrees directly overhead. Especially in telescope jargon, this method of determining coordinates is usually abbreviated as "alt/az."
Even without a compass, protractor or even a sextant, the horizonal method is very easy to use for most observers and most have at least some general idea about which direction they might be facing if they go outside so the use of this system is very common and often found on star charts. However, the obvious problem with the horizonal system is that it is based on local coordinates. Texting someone half way around the world with your coordinates, no matter how accurate, will be of no use to the other observer since if they tried to apply your coordinates to their sky, they would be looking at something completely different. We clearly need another system...
The Celestial Coordinate System
Although they did not know it at the time, the foundation for the celestial coordinate system was actually laid down by the Greeks who envisioned all of the stars in the sky to be the same distance away from Earth in a finite universe. This "celestial sphere" is of course completely fictitious but for purposes of navigating the sky, it serves as an excellent starting point since accurate distance measurements is not essential to success.
In essence, the celestial coordinate system (first used by Hipparchus) functions just like the alt/az system explained above. However, the celestial system can be used without the coordinates for a given object changing regardless of location so, scientifically speaking, it is the correct system to use. As opposed to the horizonal system that uses local coordinates as reference points, the celestial system takes into account the tilt of the Earth and applies it to the celestial sphere. But what does this all mean?
Basically, it means the position of the viewer on the Earth is no longer important since the viewer’s actual coordinates are never used as a reference point. To understand this, you have to
Star trails, a visual effect that can be mimicked by pointing a camera towards the north or south celestial pole for an extended period of time, demonstrates how the night sky appears to move around this point and not directly overhead. The result is stars rising and setting in increasingly larger arcs the farther the star is from the celestial pole.
imagine the Earth with its latitude (east/west lines) and longitude (north/south lines) lines drawn on it. Now, project these lines outward to where they would hit the celestial sphere to create a grid of lines on the sphere that exactly mirrors the lines on the Earth. Finally, imagine a star on the celestial sphere with a set of coordinates. As the Earth rotates but the celestial sphere stays in one spot, it should become apparent that, regardless of a person’s location on Earth, the coordinates of the celestial object never change. Since these coordinates never change, you can text someone anywhere on Earth and inform them to look for a set of celestial coordinates and they should be able to locate the same place in the sky without much trouble.
Like the horizonal system, the celestial system uses two measurements to describe an object in the sky. Recall that in the horizonal (alt/az) system, altitude represents the height the object is off the horizon and azimuth represents the compass direction along the horizon. In the celestial system (usually abbreviated "RA/Dec"), the two measurements are essentially the same but given different names. In the celestial system, altitude is know as declination and still varies from 0-90 degrees but uses the north celestial pole (where the star Polaris is located) as 90 degrees and the celestial equator as 0 degrees (see diagram below).
Understanding a celestial azimuth is a bit more complicated. Called right ascension in the celestial system, it uses hours and minutes instead of degrees with a maximum of value of "24 hours" for any given object (the rotational period of the Earth). Remember that on the Earth, this measurement is nothing more than longitude...all that is really happening is that instead of using the Prime Meridian as the "zero line," astronomers use the point on the celestial sphere where the sun crosses the celestial equator on the Spring (Vernal) Equinox. From there, the coordinates increase along an easterly direction all the way aro8und the celestial sphere. For instance, the star Rigel, a very bright star in the constellation Orion, has a right ascension of "05h 14m 32.272s."
Celestial Coordinates and Skywatching
Admittedly, the coordinate system is confusing to most people without an explanation by someone who knows how
to use it. In fact, for groups of people huddled together at one site to observe the sky, they will rarely use it even though the system exactly mimics the actual apparent movement of the sky. By using celestial coordinates, it is fairly easy to estimate not only where an object is in the sky but also how it will move in the future, when it might set below the horizon, etc. However, as you will see in the Optical Telescopes section, the RA/Dec coordinate system is the primary language of telescopes although its importance has diminished a bit with the advent of computer controlled and GPS equipped telescopes. The RA/Dec system is also the language of astronomy with all celestial objects identified and classified according to their declination and right ascension.
Finally, many first time skywatchers quickly become frustrated because they can never get their telescopes to move "the way they want it to." The primary reason for this lies in the fact the telescope is likely designed to use the celestial coordinate system while the observer is thinking in terms of the horizonal system. You’re effectively trying to communicate with your telescope in a different language! Take a look at the Telescope Components section of Optical Telescopes for more.
Stellar Naming Systems
Since man first began peering into the heavens, scientists have attempted to identify what they have seen. Unfortunately for us, most of these pioneer astronomers worked independently of each other and often identified the same object several different ways which creates much confusion for the amateur observer. In fact, astronomers first began classifying celestial objects for the first time over 3000 years ago. Modern technology has allowed us to reclassify objects formerly known by much less accurate observations and new catalogs have arisen to replace older ones. Through time, catalogs have been established for not only stars but star clusters, galaxies, nebulae and even black holes. Today, there are dozens of general and specialized catalogs available to astronomers for just about every object in the sky and as our capabilities to find objects continues to improve, the catalogs just get larger. Since you will encounter many of these classification systems (especially while using Stellarium or similar programs), the more common and typically larger ones are included here to assist you.
Common Star Names
While constellations usually form pictures of people or animals in various myths, star names are more of a mixed bag. Most of the brightest stars in the sky held special significance to ancient cultures and were specially named with the majority of them are related to their host constellation. For example, the star Deneb means ``tail'' and labels that part of the
The now famous Dunhuang star map, one of the oldest written star charts known in existence. Created around 700 A.D. during the Tang Dynasty, the set of manuscripts was discovered in a cave around 1900 and features over 1300 stars in clearly recognizable patterns. Image: International Dunhuang Project
constellation Cygnus (The Swan). Others describe the star itself, such as Sirius, which translates literally as ``scorching." Not surprisingly, Sirius is the brightest star in the northern hemisphere. Then there are a handful that seem utterly out of place. Lepus (The Hare) includes a star named Nihal, which means ``camels quenching their thirst.'' This is a holdover from when the constellation was known by a previous name to a non-western culture.
Quite a few star names are Arabic in origin since many ancient cultures of that region possessed excellent astronomers. A good indicator is the use of the letters al which means ``the'' and often appears in front, e.g., Algol, ``The Ghoul.'' Its inclusion has become somewhat arbitrary over time; several of these names are given elsewhere with or without the Al- prefix. Most other names in the Western tradition have Greek or Latin origins. However, a few still use non-Western names, principally Chinese. Keep in mind that practically all stars with formal names almost always owe this fact to their observation by an ancient space watching culture so they are typically the brightest in the sky and able to be seen without any kind of artificial devices. Modern astronomers study many stars too faint to see without a telescope and these are so numerous they are known only by catalog numbers and coordinates today...to continue the naming process would just not be possible.
Greek Letters (Bayer Designations)
Greek letters were first assigned to stars in 1603 by the German astronomer Johannes Bayer and are given to the brightest stars in a constellation. For instance, Betelgeuse is the brightest star in Orion and is also known by the designation Alpha Orionis. Less bright stars are given other letters in order of their apparent magnitude (their brightness as seen from Earth). For instance, Beta Orionis (the second brightest star in the constellation) is the star Rigel, and so on. Originally, this system continued to include Roman letters as well but they are rarely encountered today. It is important to note, however, that an exact match up between magnitude and the assigned Greek letter is not always perfect since Bayer also used the star’s position in the constellation when labeling it. Do not be surprised if you eventually find a beta star that is actually brighter than the alpha star...it happens quite frequently. (Click for a review of the Greek alphabet).
To organize more of the naked eye stars, English astronomer John Flamsteed (1646-1719), who created one of the great star catalogues of his day, created a list of the stars' right ascension (the astronomical version of longitude) within a constellation in the early eighteenth century. Interestingly enough, he did not actually assign a "Flamsteed number" to these stars however. In 1712, Flamsteed's comprehensive star catalog was actually published without his permission by Edmond Halley (of Halley's Comet fame) with prompting from Isaac Newton. This catalog contained the first use of a numerical system to ID the stars Flamsteed examined. Later in 1783, the French astronomer Joseph Lalande picked up
This image of the constellation Lyra includes formal names like Vega, the brightest star in the summer sky in the northern hemisphere. In addition, the brightest stars have also received Greek letters (known as Bayer Designations) and many of the stars can also be identified by Flamsteed numbers. In this case, the western boundary of the constellation is on the left of the image so 1Lyrae can be seen at the tip of the bird's wing. The numbers increase from west to east across the constellation (in this case, left to right). Lyra is also peculiar in that several notable pairs of binary stars exists there. In this case, the binary pair will often possess the same Bayer Designation but with a number after each to indicate they are a binary pair. The famous "double Double" is actually two sets of binary stars orbiting around each other.
Halley's and Newton's pirated version of Flamsteed's work, liked the numbering idea and reused it but changed the strategy for the numbers. These numbers are the ones identified as Flamsteed numbers used today. At least Flamsteed still got the credit for the catalog.
Today, the serial numbers give the star's relative location from west to east within the constellation. For example, consider Lyra, a small but prominent constellation in the summer sky (pictured above). 1 Lyrae would be the western-most star in Lyra, 2 Lyrae the next, and so on (Vega is 3 Lyrae). In general, first magnitude (and a few special) stars are commonly known by their proper names. Greek-letter names are then used until they run out, and then the Flamsteed numbers take over. However, it is not uncommon to see a particularly bright star with all three classifications. Rigel in Orion, for instance, is known by its common name but also as Beta Orionis and 19 Orionis. Finally, in a few cases there are Flamsteed numbers that sit outside the constellation they are named for since the modern constellation boundaries were not agreed upon until 1930. For instance, 10 Ursae Majoris, a binary star system, actually lies in the constellation Lynx, not Ursa Major. Rather than redesignate these stars within the new boundaries agreed on in 1930, the International Astronomical Union just let the original designations stand.
Hubble Guide Star Catalog (GSC)
The original version of this catalogue was created to support the identification and use of Guide Stars for pointing the Hubble Space Telescope. A "guide star" is typically a bright star that can assist telescopes in determining exactly where they are pointing. It is based on very precise sets of data originally assembled at the Mount Palomar Observatory (California) for the northern hemisphere and the SERC-J survey (Australia) in the south. This catalogue contains objects in the magnitude range of 7-16 and the classification was biased to prevent the use of non-stellar objects (like planets) as guide stars. For the HST, guide star alignment is typically precise down to 0.3 arcsec. Since the original version (GSC 1.0), this catalog has been updated many times with more and more precision. The GSC contains nearly 19 million objects brighter than sixteenth magnitude, of which more than 15 million are classified as stars. They are organized into 9537 geographic regions in the sky.
Bonn Durchmusterung Catalog (BD, CD, DM)
The most famous catalogue for fainter stars, the Bonner Durchmusterung (the Bonn Survey) was compiled in Germany in the nineteenth century and lists stars through around tenth magnitude. It divides the sky into declination strips (the sky's version of latitude) one degree wide and then numbers the stars from west to east according to the stars' right ascension. The catalogue name incorporates the declination of the star. Vega, for example, is also known as "BD+38 3238", which means it is the 3238th star in the declination strip between 38 and 39 degrees north. The BD covers stars to -2 degrees declination (2 degrees south of the celestial equator).
Below that, the sky could not be observed from Bonn. The rest of the southern hemisphere is covered by the Cordoba Durchmusterung (the Cordoba, Argentina Survey), or CD. The star Canopus, for example, is also known as CD-52 914, or the 914th listed star between declination 52 and 53 degrees south. BD and CD are sometimes combined as DM for Durchmusterung. As a side note, the Earth’s precession has actually moved many of these stars out of their original declination strips.
Henry Draper Catalog (HD)
The Henry Draper Memorial Catalog is the most commonly used catalogue for fainter stars (although brighter stars are also assigned). It essentially assigns a serial number for stars through roughly tenth magnitude to the east of the Vernal Equinox independently of declination. The HD catalogue, while a general tool for star names, is also a specialty catalogue and was created to list the spectral classes of over 300,000 stars. Vega, for instance, is HD 172167, Canopus is HD 45348.
Hipparcos Catalog (HIP or HIC)
The Hipparcos Astrometric Catalog, containing 118,218 stars, is one of the final products of the ESA’s HIPPARCOS satellite mission (1989-1992) and was released in June 1997. The name HIPPARCOS is actually an acronym for HIgh Precision PARallax COllecting Satellite and was the first satellite launched for the expressed purpose of astrometrical measurements. The Hipparcos and associated Tycho catalogues were constructed under by large scientific teams at the ESA from data gathered by the HIPPARCOS satellite. Although this satellite failed to reach its proper orbit due to a rocket malfunction, it was still able to provide extremely accurate information for nearly 120,000 stars making it a very large collection. The catalog and mission were named in commemoration of the Greek mathematician and astronomer Hipparchus who lived during the second century B.C.
Smithsonian Astrophysical Observatory (SAO)
In the 1960's, American astronomers merged ten earlier positional catalogues into the Smithsonian Astrophysical Observatory (SAO) star catalogue. It serially numbers over 250,000 stars according to their right ascensions in the year 1900 to ninth magnitude in 10 degree declination strips from north to south, each strip picking up where the last one left off. Vega is SAO 067174, Canopus SAO 234480. Though it enjoyed some popularity, it is no longer commonly in much use although SAO designators are still found frequently.
Double (Binary) and Multiple Star Systems
A huge number of stars are doubles or multiples locked in orbit around each other. The components of very wide (usually visual) doubles that have Greek letter names are often distinguished by applying superscripts to the Greek letters from east to west. Zubenelgenubi (Alpha Librae) is such a binary. The western of the two stars is therefore Alpha-1 and the eastern Alpha-2, even though Alpha-2 is much brighter. When the stars are close to each other, a system of Roman letters generally applied in order of discovery or of descending brightness (usually the same thing) is used. The principal component is "A," the next brightest "B," and so on. Brilliant Sirius is accompanied by a faint white dwarf. The naked-eye star is Sirius A, the dim companion Sirius B. This naming convention should not be confused with extrasolar planets however that use a similar system but use lowercase letters instead. Capital letters are reserved for binary/multiple star companions.
Some 70,000 stars today are identified as "variables" since their normal brightness frequently changes for a variety of evolutionary or observational reasons. The first found, among them the stars Mira and Algol, were quite bright and had proper names or Greek letters already by the time astronomers figured out their variability was the result of specific evolutionary processes. But, as more were discovered, astronomers needed a systematic naming system to handle all of the new additions. Since Johannes Bayer's
The Greek astronomer Hipparchus who created the first formal star catalog of the western world. Hipparchus is also credited with inventing the processes of trigonometry during his stellar observations. Many believe he also invented the astrolabe, an early astronomical viewing tool.
The binary pair HK Tauri, a set of extremely young, pre-main sequence stars first imaged by the Hubble Space Telescope Telescope in 1997. The image was the first ever to resolve a member of a binary star system with a nebulous disk of matter surrounding it. It is believed these disks ultimately lead to planet formation. Image: JPL
last Roman letter in any constellation was "Q", astronomers adopted "R" for the first variable found that did not already have a name, to which was appended the Latin possessive of the constellation name. The first such variable known in Cygnus was thus named R Cygni, the first in Aquila was R Aquilae. The sequence continued with new discoveries to the letter Z. But, as optics improved, even more variables were identified so astronomers went back up the alphabet and began using double letters, beginning with RR, then going to RS, RT...RZ, then to SS, ST...SZ, TT, TU...and finally down to ZZ. Unfortunately, the list of known variable stars again outstripped the alphabet's ability to identify them so to accommodate the ever increasing numbers the system then went back to the top of the alphabet, to AA, AB...AZ, BB, BC...BZ, CC...and so on to QQ...QZ, with J left out to avoid confusion. After an exhaustive 334 letter combinations, astronomers finally gave up when the numbers continued to rise and just used the letter "V" followed by a number like V335 Sagittarii which was the first variable star discovered in the constellation Sagittarius after QZ Sagittarii.
Messier Catalog (M)
The Messier Catalog is a famous set of astronomical objects organized by Charles Messier in his catalogue of Nebulae and Star Clusters first published in 1774. Strangely enough, this catalog was created because Messier, a comet hunter, was frustrated by objects which resembled but were clearly not comets. Given the telescopes of the day, the frequently fuzzy appearance of nebulae and globular star clusters in particular annoyed Messier to no end and his catalog was complied in order to subsequently stay away from them in his observations. The first edition covered 45 objects numbered M1 to M45 but in the end, the total list consisted of 110 objects, ranging from M1 to M110. The final catalog was published in 1781 and printed in 1784 and even today, many of the more famous objects contained in his catalog are still best known by their Messier number.
Because the Messier list was compiled by astronomers in the Northern Hemisphere, it contains only objects from the north celestial pole to a declination of about 35 degrees. Many impressive southern objects are excluded from the list for this reason. Because all of the Messier objects are visible with binoculars or small telescopes under favorable conditions, they are popular viewing objects for amateur astronomers and perhaps Messier's quirkiest legacy today is what amateur astronomers call the Messier Marathon, a well known all-night affair possible on only a few nights each year when all 110 Messier objects can be viewed and photographed in a single evening with the first objects rising in evening twilight and the last disappearing at dawn the following morning.
New General Catalog and Index Catalogs (NGC, IC)
J.L.E. Dreyer published the New General Catalog (NGC) in 1888 as an attempt to collect in one place a complete list of all nebulae and star clusters known at the time. In 1895 and 1908, he published supplements to the NGC which he called the Index Catalogues (IC) and, combined with the NGC, the total effort catalogued an astounding 13,000 deep sky objects. Nearly all of the bright, large, nearby non-stellar celestial objects known at that time have entries in one of these three catalogues. Thus, the catalogue numbers preceded by the catalogue acronyms NGC and IC are still frequently used by astronomers to refer to these objects.
Dreyer collected observations for the objects from whatever sources were available to him. While he did extensive comparisons of positions from different sources, he was working with lists of data from different observers using telescopes of vastly different sizes working under greatly different conditions. Consequently, the comparisons are therefore frequently wrong or contradictory but not through any real fault of Dreyer's; he was a careful and excellent transcriber. Many NGC/IC numbers have actually been found to refer to stars, double stars, or multiple stars and many NGC/IC positions even point to blank regions of sky. There are also many cases of two or
Messier Calatog M31 or NGC 224, the Andromeda Galaxy is one of the most photographed objects in the northern hemisphere and the most distant object that cen be seen with the naked eye. At a distance of 2.2 million light years, this "big brother" to the Milky Way is approximately 120,000 light years across and is among our nearest galactic neighbors. Image: Robert Gendler
more NGC/IC numbers clearly referring to the same object.
Some attempts have been made to re-observe all the NGC objects and to remove the errors in the catalogue but never to a great degree. The IC objects are generally smaller and fainter so have received less attention but some work has been done on them too. Unfortunately, almost all of these previous efforts have depended exclusively on the positions and brief descriptions published in the original NGC and ICs themselves.
Files and Downloads
Stellar Astronomy Supplemental I (Worksheet)
Stellar Astronomy Supplemental II (Worksheet)
Eyewitness to Stellar Evolution
Stellar Astronomy Supplemental III (Worksheet)
Files and Downloads
Stellar Astronomy Supplemental IV (Worksheet)
The Mysterious Middle of the Milky Way
Lab: Celestial Navigation
Lab: HR Diagram (AGS 381), (AES 372)
Stellar Astronomy: Take Home Exam
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