Wezen, a Deceivingly Dim Star

The Stars in the Brazilian flag are not randomly drawn. brasil1For instance, in the lower left area of the circle are six stars from the Canis Major constellation. This morning while the rest of the family either snored or dreamed, I walked the dog at an early hour under relatively dark skies. Thanks to our dim streetlamps and a waning moon. I was able to observe the fourth brightest star of Canis Major, designated as delta (δ). Named as such because δ is the fourth letter of the Greek alphabet, it also has the common name of Wezen. Deceivingly it only seems dimmer than the very bright Sirius, the alpha-dog star, because Sirius is a lot closer to the Earth.

How do we know how far away Wezen is? 

Hold a finger close and directly in front of your nose. Close one eye. Close the other eye while opening the first one. The finger seems to move against the background. If you hold the finger further away and repeat the exercise, the finger still seems to move, but not as much. Similarly for a given star, if it can be observed from two distant viewpoints along earth’s orbit around the sun, the star, will seem to be in slightly different positions against the background of more distant stars. If the distance between viewpoints is known and the angle of apparent movement is measured, simple trigonometry can help us calculate the distance between our sun and the star. The problem is that the angle is extremely small—after all, any star is a lot further away than your finger can possibly be by a factor significantly larger than the ratio of the orbit’s diameter to that of your eye-separation. A small uncertainty in angle can be amplified into a large error in distance, limiting us to measurements of only “neighborhood stars”. For more distant stars, other techniques involving Cepheid variables and type 1a supernovae have to be used. But thanks to Hipparcos, a scientific satellite of the European Space Agency (ESA) especially devoted to astrometry, parallax measurements have improved recently and are definitely accurate enough for a Milky Way star at Wezen’s distance.

In 2007, Wezen’s parallax (p) was measured to be 2.03 milli-arcsecondsParallax schematic-729x296

Given that there are 3600 arc seconds in a degree and setting the sun-earth distance at 1 astronomical unit (AU), tan p = 1/d or d = 1 ÷ tan (2.03 × 10-3/3600) = 1.02 × 108 AU.

1 lightyear = 63240 AU, so Wezen is about 1.02 × 108 AU ÷ 63 240 AU/light year = 1607 or about 1610 light years away.

How do you get absolute luminosity from distance and apparent brightness?

Due to that distance, which is far greater than the 8.61 light years that separates us from Sirius, Wezen’s apparent brightness is only 1.83.  Compared to the number line, the stellar brightness scale runs backwards. The dimmest stars have the largest positive values and the brightest have pronounced negative values.

To get the true or intrinsic brightness ( absolute magnitude) of Wezen, we can use the following formula:

M = m – 5 log (d/10),

where m = apparent brightness and d = the star’s distance from our sun in parsecs. Since there are 3.26 light years per parsec,

M = 1.83 – 5 log(1607÷3.26÷10) =   6.63

That’s a lot more intrinsically bright than Sirius, which has an M value of +1.42. It is Sirius’ proximity to us that makes it the 2nd brightest star in the sky after our sun and puts its apparent brightness at  – 1.47. If you imagine them to be both at Sirius’ distance from Earth, by doing the math you realize that Wezen would have an apparent brightness of 9.52, which would be almost as bright as a half-moon.

Wezen
By Sephirohq – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15613651

Using Two Measurements To Learn About Wezen’s Nature

Now if you rely on one other measurement for Wezen, something even more startling will be revealed. Its color is yellow, and with spectroscopic analysis of the lines from its excited atoms, it is classified as F8Ia, which gives away its surface temperature.

If you then plot absolute magnitude versus spectral class for various stars you get astronomy’s equivalent of the “periodic table”. It’s called the Hertzsprung–Russell diagram and it reveals a star’s stage in its evolution.HR-diag-instability-strip.svg

Using F8 as its x coordinate (each class has 10 subdivisions, zero through 9, so F8 is close to G’s lower bound) and its absolute magnitude of -6.63 as its y coordinate, we end up with a coordinate point on the supergiants line on the instability strip.

Our sun’s class of G2 and absolute magnitude of 4.83 place it on the main sequence, which is why we are still alive. Notice however that the sun’s spectral class is telling us that its surface is actually a little warmer than Wezen’s. If Wezen’s luminosity is so much greater than that of the sun, Wezen has to be a lot bigger, but its energy is spread thinly over its large surface area. But with more mass, gravity is a lot stronger, driving Wezen’s core temperature exponentially higher. This accelerates its rate of fusion. To make a long story short, Wezen is only 10 million years old and has already stopped fusing hydrogen, whereas the sun has celebrated its 4.6 billionth birthday. Moreover, the sun will also stay on the main sequence long enough to double its present age.

It seems that Wezen has already started to expand. As it fuses helium, it will become a red supergiant and eventually go supernova within a mere 100 000 years. When that happens, in our night sky, Wezen will appear almost as bright as Sirius and brighter than every other star. It’s comforting to think that maybe our descendants will walk with their dogs early one morning and marvel at it.

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Visions from the Atacama

atacamaFormsThe driest non-polar region in the world is Chile’s Atacama Desert. Some areas have gone 173 months without rain. At about a latitude of 20 degrees south of the equator, prevailing southeast trade winds carry moisture into the the eastern slopes of the Andes. But as the warm air ascends,  it encounters lower atmospheric pressure and expands at the expense of its own internal energy and cools. Precipitation ensues. As the remaining dry air descends over the other side of the Andes it compresses as it descends and heats up. Meanwhile the western shoreline is unusually deep, keeping the Pacific waters at that spot quite cold. There is also a cold current from the south, preventing cold onshore winds from delivering any moisture to the area. Dry air and no clouds for years on end are an astronomer’s fantasy, and it is why the Atacama Desert is the location of the European Southern Observatory’s Very Large Telescope facility (VLT). VLT consists of Antu, Kueyen, Melipal and Yepun, four telescopes that can be operated independently or in harmony to achieve a better resolution.

VLT-MilkyWaySet_6016-Crop-net
In the foreground VLT’s Antu, Kueyen and Melipal with the Milky Way in the sky.

VLT has been producing a large volume of sharp, beautiful images and noteworthy science. In 2009 VLT revealed that the star Betelgeuse has a vast plume of gas almost as wide as our Solar System and a gigantic bubble boiling on its surface. The star’s atmosphere is apparently constantly stretching out into space and then retracting, losing some material in the process.

Presently (late August, early September) from the southeastern part of North America, Betelgeuse is visible before dawn in the southeastern sky.  A star in Orion’s “right bicep”, it’s a red giant with an inconsistent peachy color.  A red giant that is massive enough to go supernova, Betelgeuse is within our galaxy, only 642 light years away. When it explodes some time within a thousand years, it will be bright enough to be visible on Earth in broad daylight. The plume observed from VLT reveals that Betelgeuse is asymmetric. All red giants shed material, but Betelgeuse is not spewing it out equally in all directions.

Two years later, again using the VLT,  astronomers discovered a surrounding nebula, bigger than Betelgeuse itself, stretching 60 billion kilometres beyond the star’s surface, or approximately 400 times the Earth-Sun distance.  The visible part of the nebula turns out to be made up of silicate and alumina dust. When we looked at the cosmic origins of the chemical elements, we learned that silicon is formed from a massive star capable of generating the necessary temperature and density. A red giant of Betelgeuse’s dimensions satisfies those requirements. With regard to the prominence of aluminum, it suggests that supernovae are not the only source of that element.eso1121a

Another telescope in the same desert is the Atacama Large Millimeter/submillimeter betelgeuse-captured-by-almaArray(ALMA). ALMA has just come up ( in 2017) with the highest-resolution image of Betelgeuse to date. It gives us a clearer look at its asymmetry.

A question that may arise is how can a star that’s approximately 20 times more massive than our sun reach the supernova stage in less than 10 million years, while our star, which although isn’t massive enough to go supernova, is still billions of years away from reaching the red giant stage? A very rough calculation will shed some light on this. Twenty times the mass means that there will also be 20 times the fuel available; however for stars of that size, the luminosity ratio is roughly equivalent to the mass ratio of the main sequence stars raised to a power of 3.5. In other words Betelgeuse has more fuel, but the added heat from the much stronger gravity makes the fuel at the core fuse at a prodigiously higher rate! The time that Betelgeuse spent on the main sequence was only 20 / ( 203.5) of the time that will be spent by our sun. Our sun’s life span on the main sequence is about 10 billion years, but Betelgeuse only spent 10×109[20 / ( 203.5) ] or about 6 million years, if we respect significant figures.

atacamaThe Atacama desert was named after a group of people who settled the northeastern border of the desert at least as far back as 500 AD. The Atacama people, known as atacameños or atacamas, used Rapé  smoking ceremonies, which they believed brought them closer to the gods. It’s fitting that their desert now hosts instruments and minds that try to get closer to distant secrets of the universe.

 

 

 

 

 

Other Sources:

http://www.eso.org/

http://www.eso.org/public/news/eso1121/

http://www.eso.org/public/news/eso0927/

Salaris, Maurizio; Santi Cassisi (2005). Evolution of stars and stellar populations. John Wiley & Sons. pp. 138–140

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