The Story of Altair

Star’s Story

There are many myths in many different cultures that have different stories regarding the constellations in the night sky. The main myths we hear today stem from Asian and European backgrounds.

Probably the most famous myth about Aquila, the constellation which encompasses Altair, involves Prometheus, a Titan god. Prometheus was a Titan gods who eventually became an adviser to Zeus. Prometheus was kind towards humans and seeing how they suffered because they had no fire, ended up stealing a ray from the Sun which he smuggled down to earth for humankind to use. Zeus did not believe that humans deserved the gift of fire, and was furious that Prometheus help them without his permission. Prometheus was consequently chained to the side of a mountain, stripped of his clothes, and was continually attacked by Aquila. Because Prometheus is a Titan god and therefore immortal, his wounds healed up every night only to be opened up again the next day by Aquila. After many years of this continued punishment, Prometheus was saved by Hercules who supported this kind deed Prometheus had done for mankind. Hercules then used his bow and arrow, shot and killed Aquila who was then placed by Zeus to soar to the heavens.

In India, Altair along with the two stars beside it, Beta and Gamma (Tarazed and Alshain), are said to be the celestial footprints of the god Vishnu.

Altair is separated from the similar-looking star Vega, in the constellation Lyra, by the starlit band of the Milky Way. In Asia, this hazy band across the sky is known as the Celestial River. One common story in China, Japan, and Korea, is that a young herdsman (Altair) falls in love with a celestial princess (Vega), who is in charge of weaving the fabrics of heaven. Vega fell so madly in love with the Altair that she neglects her duty of weaving the heavens. Vega’s father, the Celestial Emperor, declares that Altair must stay away from his daughter, on the opposite side of the Celestial River. The Emperor finally listens to the princess’s pleas and allows Altair to cross the Celestial River once per year, on the seventh day of the seventh month. In Japan, Altair is known as Hikoboshi, and Vega as Orihime (or Tanabata).

Location, Names, Distance, and Magnitude

Altair is the 12th brightest star in the night sky and is part of the Aquila constellation. Aquila is the 22nd largest star constellation in the sky, occupying an area of 652 square degrees in the fourth quadrant of the northern hemisphere. Aquila is visible between the latitudes +90° and -75°. Altair is also part of The Summer Triangle made with the help of Vega and Deneb. These three stars form a triangle and are the brightest stars in their separate constellations. Aquila is Latin for eagle and is visible from the northern hemisphere in the summer months because of the path the Earth takes around the Sun. Altair is visible from Calgary during these months. We can’t quite see Altair in the winter months because the star is located in the direction of the sun during the day and is consequently blocked out by the suns brightness. In the month of July, Altair is visible directly east at 10 p.m in mid-northern latitudes. Another name for Altair is Alpha Aquilae.

Altair is approximately 16.6 light years away or 5.1 parsecs. Because Altair is relatively close, the distance to it can be calculated using stellar parallax. Measuring the angle Altair moves in the sky as we move in orbit around the sun, combined with the known distance we travel, we can use this to calculate the distance away using basic trigonometry. With an apparent magnitude of 0.76 and absolute magnitude of 2.2, Altair is the 12th brightest star in the entire sky. The sun’s absolute magnitude is -26.7 which means that at the distance Altair is to us, if the sun took its place, we wouldn’t be able to see it. Apparent magnitude is the brightness of a celestial object as observed from earth, while the absolute magnitude is how bright that object would appear at a distance of 10 parsecs. These measurements mean that if Altair were substituted for our sun, at the distance the sun is now, life on Earth wouldn’t survive because Altair shines with 11 times the sun’s visible light.

Physical Characteristics

There are trillions of stars throughout the universe and each and every one of them has a different set of physical characteristics. These characteristics include things like color, composition, size, temperature, density, shape, and mass.

For example, some of the physical characteristics of the star Altair include:

 

 

 

 

 

 

 

 

Mass:  1.7 M

Effective temperature :7550K

Size: 1.83 R☉

and the physical characteristics of our sun include:

 

 

 

 

 

 

 

 

 

Mass: 1.0 M

Effective temperature: 5800k

Size: 1.0 R☉

When we compare the measurements of these two stars we can see that  Altair is about 1.7 times more massive then our sun, 1.83 times larger, and about 1.3 times hotter at its surface.

While it might be true that stars all have different physical characteristics these characteristics are limited to: certain colors, sizes, mass’s, shapes, densities and temperatures.

When we compare the sun to Altair, the size, mass and temperature of Altair are all greater than the mass, temperature and size of the sun. The reason being, these physical characteristics are all related to one another and the overall structure of the star. For example, there is a direct link between the luminosity of a star (the amount of energy it emits) and its mass. There is also a direct link between the apparent color of a star and its temperature. This is shown using something called the black-body spectrum.

Image result for blackbody spectrum

What is a a black body?

A blackbody refers to an opaque object that emits thermal radiation. A perfect blackbody is one that absorbs all incoming light and does not reflect any. Stars are also blackbody radiators and most of the light directed out of stars is absorbed. This is why different stars have different colors.

As you can see there are three values on the graph above: intensity, temperature and wavelength. When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. This means that if Altair is 1.3 times hotter then the sun then it should emit a lot more radiation and be noticeably different in color.

So how do we find out what color Altair is?

This relationship is called Wien’s displacement law and is useful for the determining the temperatures of hot radiant objects such as stars, and indeed for a determination of the temperature of any radiant object whose temperature is far above that of its surroundings.When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

By rearranging this formula we can take the temperature of our sun, 5800k, and use it to determine its peak wavelength, 499.65517241379314 nano meters. This means the peak wavelength is for greenish-yellow light but the Sun is putting out similar amounts of blue and red light. The overall effect is approximately white light with a yellow tint, making the sun appear yellow/orange.

Altair is 7550k and its peak wavelength is 383.84105960264907 nano-meters. This means that the peak wavelength is purple/ ultraviolet light but Altair is putting out more blue light then red light, making it appear blue in the night sky.

Classification

The image below depicts the spectrum for Altair and is also what astronomers use to classify stars. Astronomers classify stars based on their spectral characteristics which are the qualities of the light we obtain from a star. Looking at the light from a star can  tell us a lot about it thanks to what physicists have discovered about the way light interacts with other things in the universe at the atomic level. Light created in the star interacts with other elements within the star and creates dark absorption lines on the stars spectrum. Each line represents the ion of a specific chemical element and the darkness of the line represents the abundance of this element. This spectrum not only shows the composition of the star but it can also be used to determine its density, temperature, size, its mass and many more things, including the movements of nearby objects.

Spectroscopy is one of the fundamental tools which scientists use to study the universe and this is why we use spectrum to classify stars. Spectral class ranks stars based on the density and temperature of their photo sphere and most stars are currently classified under the Morgan–Keenan system. Each star is divided into classes O,B,A,F,G,K,M with O being the hottest and M being the coldest and, then again into sub classes from 0-9 with 0 being the hottest and 9 the coldest. The suns spectral type is GV2 and Altair’s spectral type is A7. This provides us with a measure of how hot and dense the photo sphere of the sun and Altair are relative to other stars in the universe that we have observed.

Temperature
O 30,000 – 60,000 K Blue stars
B 10,000 – 30,000 K Blue-white stars
A 7,500 – 10,000 K White stars
F 6,000 – 7,500 K Yellow-white stars
G 5,000 – 6,000 K Yellow stars (like the Sun)
K 3,500 – 5,000K Yellow-orange stars
M < 3,500 K Red stars

Using this scale we can see that there is a full spectral class separating the sun and Altair, as they’re characterized by significant physical differences.

 

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The above image is the spectrum for the sun. We can see that it is composed of sodium hydrogen, magnesium, calcium, iron,  and oxygen . The spectrum for Altair shows that it is made of: hydrogen, magnesium, argon, iron, mercury and sodium . So the sun and  Altair each contain two elements that the other doesn’t have. Also, both these stars have varying amounts of each element. We can tell from these spectra that Altair is richer in hydrogen, magnesium and iron while the sun is richer in oxygen and calcium. With all the information contained within these spectrum it is no wonder why astronomers use these to classify stars. It also shows us how easy it is to learn about what stars are made of and how that relates to their temperature, mass, size, etc.

 

HR Diagram

 

With a visual magnitude of .77, Altair is a main sequence star that is almost 11 times more luminous than the sun, while having 1.7 times the sun’s mass and twice the sun’s diameter.  Its spectral type is A7, which makes sense considering its white appearance. It’s type A classification is due to Altair’s strong hydrogen lines, and lines of ionized metals. The sun, a main sequence star with a G2 classification, has strong H and K lines of Ca, weaker Hydrogen lines and neutral metals.

Altair is currently just under one 1 billion years in age (<1Gyr), and it’s expected to live 4.3 billion years. In comparison, the sun is already 4.6 billion years old, and its lifetime is predicted at around 10 billion years (leaving us around another 5 billion years until the fuel runs out).

 

Evolution

As a main sequence star with a (low) mass similar to that of the sun, Altair goes through a lengthy evolutionary process that ends with a gentle transition into being a burned out black dwarf.

It currently presents in stage 7, as a white dwarf whose core is slowly fusing hydrogen and helium, through nuclear fusion called core hydrogen burning. Once hydrogen in its core has become depleted, the inner helium rich region becomes larger. Eventually the inner helium core contracts and this shrinkage releases gravitational energy, which increases the central temperature and heats the outer burning layers. The hydrogen shell around the core begins to burn at temperatures between 10^7 and 10^8 K while the inner helium core sits like ash in the center. At this stage, the star burns brighter in response to the lack of fire in its center.

Eventually, Altair’s surface temperature drops as its surface area dramatically increases. At this stage the star is called a subgiant. With an unbalanced, shrinking helium core the star will eventually become a red giant. From main sequence to red giant, it would take Altair about 100 million years. As a red giant, the star is huge. Its cores density is enormous: it’s compacted helium gas is around 10^8kg/m^3 in the case of the sun.

Helium begins to burn the core of the star, temperatures reach about 10^8 K, which is needed for the helium to fuse to carbon and the central fires reignite through the triple-alpha process. In the core of a red giant (supported by electrons) pressure operates independent of temperature, and as burning begins again the temperature increases but there is no change in pressure. The core can’t respond fast enough, the temperature quickly spikes, in a condition called the helium flash, wherein the helium aggressively burns for a few hours allowing the core to expand, density drop, and internal and external pressure regain balance. The helium is quickly consumed and the ailing star returns to the extremely high temperatures, with a shrinking nonburning carbon core.

So now, the star would contain a contracting carbon core surrounded by a hydrogen burning shell, and the star becomes a more swollen red giant again. Close to the end of it’s nuclear burning lifetime, stars near the mass of the sun never exceed temperatures of 600 million K.

Eventually, Altair’s inner density reaches a point where it can no longer be compressed (10^10 kg/m^3). With the contraction of the core no longer happening, the temperature of the core stops rising and once carbon is formed the fire goes out. In it’s final stages, the stars outer-core burns hydrogen and helium while the inner core at its final high-density state has an increasingly intense nuclear burning at the same times as the envelope continuing to expand and cool. The star now consists of two unique parts: the small dot of carbon ash and the outermost layers that are still fusing helium into carbon and oxygen. As the core contracts and heats up to deplete its last bit of fuel, it becomes so hot its ultraviolet radiation ionizes the surrounding cloud, forming a planetary nebula.

The central star eventually cools, while the expanding gas cloud eventually disperses into interstellar space. The carbon core continues to evolve, becoming more visible with the disappearance of the envelope. The core has shrunk to about the size of earth, shining only by heat with its white-hot surface, now called a white dwarf with a temperature of 6900-8500 K. The star now fades and cools over time, slowly becoming a burned out ember called a black dwarf. The corpse of Altair will remain cold and dense as it slowly fades away.

For more on what happens towards the end of Altair’s lifetime, check out this PBS Crash course on White Dwarfs and Planetary Nebulae.

Sources Consulted:

Chaisson, Eric, and S. McMillan. Astronomy Today. Upper Saddle River, NJ: Prentice Hall, 2002. Print.

https://www.nsf.gov/news/news_summ.jsp?cntn_id=109612

http://www.daviddarling.info/encyclopedia/S/Summer_Triangle.html

http://www.constellation-guide.com/constellation-list/aquila-constellation/

http://earthsky.org/brightest-stars/altair-the-bluish-jewel-of-the-eagle

http://www.heavens-above.com/myth.aspx?con=aql&lat=0&lng=1&loc=Unspecified&alt=0&tz=CET&cul=en

http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html

https://osr.org/blog/astronomy/altair/