Viewing the “Pup”

Categories: Observing
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Published on: January 19, 2012
Sirius A and B
A white dwarf star in orbit around Sirius (artist's impression). NASA image

One of the more prominent stars in the night sky and the brightest is α Canis Majoris, better known as Sirius or the “Dog Star. At a distance of 8.6 lightyears, Sirius is the fifth closest known star. It is a slightly bluish, white main sequence dwarf star of spectral and luminosity type A0-1 Vm, with a visual magnitude of -1.58 making it nine times brighter than a typical 1stmagnitude star. It is a relatively large dwarf with a mass a little more than twice that of Sol, a diameter about 1.7 times larger than Sol, and a luminosity 21 times that of our home star. Sirius is approximately 250 million years old and will exhaust the hydrogen in its core in about 750 million years and will eventually become a white dwarf similar to its companion, Sirius B, also known as the “Pup”.

Comparison of the Sizes of Stellar Remnants
It is difficult compare the relative sizes of black holes, neutron stars, white dwarfs, the Sun, and red supergiants on the same sheet of paper. Their size differences are so vast. If you represent the diameter of a three-solar-mass-black hole or its event horizon by a dot 1/64 of an inch across, the size of a 1.4-solar-mass neutron star would be only slightly larger. But the size of a white dwarf would be about one foot, that of the Sun would be 38 yards, and that of a red supergiant, such as Betelgeuse, would be 22 miles. The true sizes of these objects range from 10 miles for a three solar mass black hole to 875,000,000 miles for Betelgeuse.

Sirius B is a white dwarf of type DA2-5 or A2-5 VII. It is 360 times fainter than Sol and compared with our Sun, it has nearly the same mass but less than one percent of its diameter. Sirius B$s diameter of about 11,700 km (about 7,300 miles) is about 92 percent of Earth$s diameter. Its mass and diameter are consistent with the theoretical size for a carbon-core white dwarf, one that may have evolved from a 5 Solar-mass, B-type main-sequence star about 125 million years ago, and after 100 million years as a giant star burned up its supply of hydrogen and threw off its outer layers. So, Sirius B was once brighter than Regulus A, currently a B7 main-sequence star. While now tiny compared to main sequence stars, white dwarf stars are actually intensely hot, but without the internal heat of fusion to keep them burning, they gradually cool and fade away.

The separation of these two stars varies from 3 arc seconds to 11.5 arc seconds over a period of almost 50 years. Under excellent viewing conditions a 60mm refractor can split double stars of similar magnitudes separated by 2 arc seconds. So why is it so difficult to split Sirius with your telescope? The brilliance of Sirius A simply overwhelms the much fainter Sirius B.

So how do you split Sirius? You could use the same technique that Alvin G. Clark used to inadvertently discover the Pup. In January, 1862, Clark was testing a new 18 ½-inch refractor. The telescope was pointed towards the edge of a building where Clark knew that Sirius would be emerging any minute from that position. To his surprise the first star that he saw was the companion, Sirius B. Now you won$t need a huge Clark refractor to see the Pup, again, in theory, a 60mm telescope should be able to split the pair. But, because of the brightness of Sirius A, you will probably need at least a 4″ or 6″ telescope to view Sirius B.

Before you try this method it is helpful to know exactly where Sirius B currently is. If you imagine that Sirius is at the center of the face of a clock Sirius B would be at the 8:45 position. If you are using a refractor or a similar telescope that gives an upright but reversed image the Pup would appear in the eyepiece in the 3:15 position. A slight modification of this method is to center Sirius in the eyepiece and then shift the telescope so Sirius A is just outside the field of view in a position that is 180° away from Sirius B. Or said a different way, move Sirius A out of the field of view on the edge of the field opposite the direction of the tiny white dwarf.

Hexagon Mask for a Telescope
Example of a hexagon mask

Another method that can be used to observe Sirius and other close multiple star systems is to create a hexagonal mask for your telescope. If you have a telescope with a corrector plate, you simply need to cut a piece of cardboard or project board the diameter of your corrector plate, then inside the circle cut a hexagon, leaving one inch at the vertices. You should glue three 1/2 inch dowell rod tips on the inside of the mask to prevent the mask from scratching your corrector plate and you may want to glue a fourth dowell rod tip to the outside of the mask to act as a handle to make it easier to rotate the mask.

If you have a Newtonian or a refractor, you can omit the inside dowell tips, but you$ll need to add a flange to the edge to secure the mask to your telescope. Something akin to an oatmeal box lid. The hexagon mask will eliminate the airy disks around the bright component and change them to a bright star with six spikes. This simple homemade tool will make it possible to view close and high contrast binary stars.

There is another technique that may prove successful. Set up your telescope before it gets dark. Let the optics cool. Try to find Sirius in the approaching twilight. Keep observing with Sirius in the center of the field as it gets darker. With the brilliance of the Dog Star subdued by the bright sky it is possible to view the Pup.

No matter what method you use to try to split Sirius or other multiple star system, there are several tips which can be very helpful:

  1. Know where the secondary star(s) will be in relation to the primary star.
  2. Only try to split the stars when the system is on or near the meridian, as nearly overhead as possible. This reduces the amount of atmosphere that the star$s light passes through.
  3. Try it only on the steadiest of nights.
  4. Use as much magnification as seeing conditions will allow.
  5. If one of the stars is intrinsically brighter that the others, move that star out of the field of view in the opposite direction of the dimmer star(s).
  6. If you have a telescope with a spider, such as a Newtonian, be sure you don’t have the secondary star(s) aligned with one of the diffraction spikes.

Observing the Pup is not easy. If you don’t succeed at first, don’t give up. The separation between Sirius A and B is about 8 arcseconds and will continue to grow to 11.5 arcseconds in 2025, making it a little easier to see the Pup over the next 17 years.

Sirius A & B
Sirius A & B from the McDonald Observatory

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