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Cosmic Challenge: Sirius and the Pup

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Cosmic Challenge: Sirius and the Pup

February 2022

Phil Harrington

This month's suggested aperture range:
6- to 9.25-inch (15- to 24-cm) Telescopes












Sirius and
the Pup


06h 45.1m

-16° 42.8'


-1.46, 8.30

see below


Last month, I offered what many believe is one of the sky's greatest challenges, the Horsehead Nebula. This month, I am back with another classic test.


Ask an amateur astronomer to name binary stars that are difficult to resolve and one of the most common responses will probably be Sirius, in Canis Major. While there are more difficult targets, Sirius is always a perennial favorite. The challenge comes not from the close separation of the two stars in the system, however. Rather, the challenge here is from the extreme difference in the two stars' magnitude. Sirius A, a type-A1V star, is about twice the mass of our Sun, while Sirius B is a white dwarf, the last stage of a star that was once approximately five to six times our Sun's mass. Sirius B, also known as "The Pup," is believed to contain about 0.5 solar mass at present, the rest having exhausted into space over 100 million years ago. Today, all we see left from that once mighty star is the incredibly hot remnant core, a white dwarf.


Above: Evening star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington


Above: Sirius and the Pup's positions 2008-2056. Note that south is up and east is to the right, matching the view through an inverting telescope.


Credit: Chart adapted from Cosmic Challenge by Phil Harrington


The Pup's existence was first detected in 1844 when German astronomer Friedrich Bessel noticed that Sirius wobbled ever so slightly against the background sky. From this, he reasoned that the gravity from an unseen companion must be causing the star's odd behavior. Nearly two decades later, Bessel was proven correct. American telescope-maker and astronomer Alvan G. Clark spotted the little companion on January 18, 1862, while testing an 18½-inch refractor, then the world's largest, that he had made for the University of Mississippi. Clark had selected Sirius for testing the level of chromatic aberration in the telescope; it is unlikely that he was specifically looking for the companion or that he even knew of Bessel's conclusions.


Sirius B measures no more than twice the Earth in diameter, yet its mass is nearly equal to the Sun's. Were we able to scoop up a teaspoon of Sirius B, transport it back to Earth and place it on a scale, we would find that it weighs several tons. Such is the stuff of white dwarfs. Eventually, Sirius B's energy emissions will ebb and cool, leaving just a cinder of super-dense carbon. What do you get when you compress carbon under extreme conditions for eons of time? You get a diamond. Yes, eventually, Sirius B will become a diamond roughly the diameter of Earth!


Spotting Sirius B takes the just the right combination of excellent optics, both in the telescope as well as in the sky. If the sky's optical behavior -- that is, seeing -- is uncooperative, then even the finest telescope optics will fail to resolve the Pup. Fortunately, things are getting easier. That's because Sirius B is heading toward the apastron point in its 50-year orbit around Sirius A. Over the course of half a century, the separation of these two stars varies from 3 arc-seconds to 11.5 arc-seconds. The pair appeared closest in 2000 and have been widening ever since. Both will continue to grow apart until next year, after which, they will close on each other. The table below lists the separation and position angle of Sirius B over the next several years.



Position Angle (°)

Separation (")

















Even when the separation is widest, seeing the Pup takes strategy. Beyond steady skies and high magnification, determine where the companion should be in the view relative to Sirius itself, and then move Sirius just out of the field. Keep in mind that, however, that depending on your eyepiece, edge distortions could distort the Pup out of existence. Therefore, many use an occulting bar across the center of the field to hide the glare of Sirius. Rotate the eyepiece around until it matches the Pup's position angle, which is currently toward the east-northeast. Incidentally, if you are using a Newtonian or Cassegrain reflector that has a spider mount holding the secondary mirror in place, double check that a diffraction spike from the spider does not inadvertently cover the star.


Here is a great capture of Sirius and the Pup by CN'er Thomas Ashcraft, which he shared in the Deep Sky Observing forum last January. He imaged the pair back on January 24, 2012, using a Celestron C14, ZWO 224 MC camera, and a Luminence filter. He also created a wonderful short video showing the visual appearance. You can view that on his YouTube channel here.



Above: Sirius and the Pup. Photo copyright by Thomas Ashcraft. Used with permission.


Give the Pup a try when Sirius is highest in the southern sky, but again, wait for those nights when exceptional seeing. Be sure to begin your hunt before the sky fully darkens, since a twilight sky will help absorb some of Sirius's glare. Sirius B is magnitude 8.5, so the sky does not have to be darkened fully to see it. Set your telescope up before the Sun goes below the horizon, let the optics cool to ambient temperature, and then as you are waiting for twilight to wane, focus on Sirius and see what you discover.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's challenge.  Contact me through my website or post to this month's discussion forum.


Until next month, remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2022 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.



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