Example 2: How much brighter is the Sun than the full Moon? For this we recall from the table above that the Sun has an apparent magnitude of It is important to remember that magnitude is simply a number, it does not have any units. The symbol for apparent magnitude is a lower case m ; you must make this clear in any problem. What does the fact that Sirius has an apparent magnitude of Another way of thinking about this is to ask why is Sirius the brightest star in the night sky?
A star may appear bright for two main reasons:. The apparent magnitude of a star therefore depends partly on its distance from us. In fact Sirius appears brighter than Betelgeuse precisely because Sirius is very close to us, only 2. The realisation that stars do not all have much the same luminosity meant that apparent magnitude alone was not sufficient to compare stars.
A new system that would allow astronomers to directly compare stars was developed. This system is called the absolute magnitude, M. The absolute magnitude, M , of a star is the magnitude that star would have if it were at a distance of 10 parsecs from us. A distance of 10 pc is purely arbitrary but now internationally agreed upon by astronomers. This logarithmic scale is also open-ended and unitless. Again, the lower or more negative the value of M, the brighter the star is.
Absolute magnitude is a convenient way of expressing the luminosity of a star. Once the absolute magnitude of a star is known you can also compare it to other stars. As you may recall from the section on astrometry, most stars are too distant to have their parallax measured directly. Nonetheless if you know both the apparent and absolute magnitudes for a star you can determine its distance.
Let us look again at Sirius and Betelgeuse plus another star called GJ How far away is GJ 75? It is an unusual star in that its apparent and absolute magnitudes are the same. The reason is that it is actually 10 parsecs distant from us, so by definition its two magnitudes must be the same. What about Sirius? Its apparent magnitude is lower therefore brighter than its absolute magnitude. This means that it is closer than 10 parsecs to us.
Betelgeuse's apparent magnitude is higher therefore dimmer than its absolute magnitude so it would appear even brighter in the night sky if it were only 10 parsecs distant. Is there a quick way of checking whether a star is close or not? This value is negative and Sirius is closer than 10 pc. This value is positive and Betelgeuse is more than 10 pc distance. Astronomers use the difference between apparent and absolute magnitude, the distance modulus, as a way of de terming the distance to a star.
A formal derivation of this equation is given in the next page on luminosity. You should be comfortable in solving this equation given any two of the three variables. Let us know look at how you can solve some examples. Example 3: Given m and d , need to find M. What is its absolute magnitude?
Remember in solving magnitude equations log refers to logarithms to base 10 and not natural logarithms or ln. Example 4: Given m and M , find d. Astronomers think the outer layers of the star expand slowly for several years and then shrink again, so the surface area alternately increases and decreases, and the temperature rises and falls, making the star brighten and dim. Red supergiants pulsate this way because their atmospheres are not quite stable.
When the star is smallest, the atmosphere absorbs a bit too much of the energy passing through it, so the atmosphere heats and expands. As it expands, it becomes thinner. Energy then passes through the outer layers more easily so the gases cool, and the star shrinks again. A Harvard-Smithsonian Center for Astrophysics Press Release announcing the first direct image of a star Betelgeuse , suggests that this event holds new implications for stellar research.
Researchers said that this new picture of Betelgeuse suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. Dupree explains that the spot might change position over time.
Dupree and colleagues using the International Ultraviolet Explorer found a day period, during which the star oscillates, or "rings" like a bell. The oscillations, thought to be caused by turbulence below the surface of the star, might cause changes in the bright spot. Future oscillations might spur other bright spots on different regions of the star's surface, causing it to wink on and off like blinking lights on a Christmas tree, said Dupree.
Alternatively, the spot might move systematically across the star's surface, which would imply that the star has magnetic fields strong enough to hold the bright spot's hot gas in position. Either scenario would lead astronomers to re-think completely current ideas of how stars evolve.
Such instruments would greatly improve the resolution of structures on stellar surfaces. The Class of Semiregulars. Betelgeuse is classified as a semiregular variable type SRc.
The distinctions between Mira stars, Semiregulars, Irregulars, and RV Tauri stars is a bit foggy and are usually discussed together due to their similarities. Since Betelgeuse is classified as a semiregular variable our discussion will focus on these types of stars.
Semiregular variables are post main sequence cool red giant and supergiant stars. There are four groups of semiregular variables SRa, b, c, and d. The descriptions of these classes follow:. SRa — These stars have periods longer than 35 days, V amplitudes less than 2. They are similar to Mira stars in their periodicity but are distinguished by their smaller amplitude.
A typical member of this subgroup is the star Z Aqr. SRb — These are also small amplitude stars but with less regularity in their light variations than the SRa stars. The SRb stars have periods longer than 20 days and V amplitudes less than 2. Although the form of a period is evident, it becomes inoperative from time to time.
SRc — These are the Supergiants which are extremely luminous stars. They have low amplitudes and occasional standstills. Betelgeuse is classsified as an SRc type variable. SRd — Yellow giants and supergiants of spectral classes F-K: not a very homogneous group. They are much hotter than the other semiregulars and show irregularities at times. Betelgeuse is a difficult star to observe due to its redness, lack of well-placed i.
It is also best to compare a red star with another red star at about the same altitude. Although a challenge to observe, it is encouraging that Betelgeuse is so bright and easy to find.
Observers could create an observing program in which you observe Alpha Ori in the coming months once every week. Try not to compare your observations with those in our WebObs search or Light Curve Generator to avoid bias in your estimate and see if you can really detect the light variability in this supergiant.
Photoelectric photometry observers are especially encouraged to observe this star of small amplitude. Next year, with the first sight of Orion in your sky, be sure to observe Betelgeuse again. The name "Betelgeuse" is a corrupt derivative of the original Arab nomenclature due to repeated transcriptions and transliterations over many centuries. According to George A. The "original" meaning of this word designates a black sheep with a white spot in the center of its body, however scholars have interpreted the meaning to come from a similar Arabic word, jauz , which means "the center of anything" or "the central one".
Davis believes that the true origin of the name is Yad al-Jawza' , "the forefoot of the white-belted sheep," one diacritical point missing under the ya in Yad resulting in the transliterated syllable bad or bed.
In the Northern Hemisphere, it can be seen rising in the east just after sunset during January. By mid-March, the star is due south in the evening sky and visible to virtually every inhabited region of the globe. In large cities in the Southern Hemisphere e. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after the Sun sets.
The description above is based on the Betelgeuse entry in Wikipedia. For more information about Betelgeuse, see Stars Jim Kaler. All rights reserved. They may not be reproduced, published, copied or transmitted in any form, including electronically on the Internet, without written permission of the author. All images are digitally watermarked. All Rights Reserved.
0コメント