Betelgeuse is BIG (and variable)

Betelgeuse is a bright star in the shoulder or armpit of the constellation Orion. It is easily visible in February and March from Vancouver around 10 pm, close to the southern Horizon.

Betelgeuse in Orion
Betelgeuse in the constellation Orion. Near the Southern Horizon from Coquitlam on Feb 18th, 2020

If you follow astronomy news at all you’ll have heard that Betelgeuse has recently been dimming. Betelgeuse is a variable star whose brightness is known to periodically rise and fall, but it’s recent dimming has been quite extraordinary. Especially exciting is speculation that Betelgeuse may explode as a brilliant supernova which would be visible even in daylight. Unfortunately the latest measurements have shown that Betelgeuse may be brightening again,

Less often reported is that Betelgeuse is BIG! – not too surprising given that it is classified as a red supergiant. So how big is it?

  • It is so big that its diameter is about 1300 times that of our sun.
  • It is so big that if placed at our Sun’s location, the outer edge of its photosphere would reach out to Jupiter.
  • It is so big that it was the first star to have its angular diameter measured. Stars are so far away that they appear as pinpoints of light whose angular diameter cannot be determined. But Betelgeuse was big enough that 100 years ago, in 1920, it was the prime candidate to submit to measurement of its angular diameter.
Betelgeuse Sun Size Comparison
Size Comparison of Betelgeuse and our Sun
Betelgeuse Overlay on Solar System
Betelgeuse overlaid on the Solar System
Image credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Michelson and Pease used the 2.5 m Hooker telescope at the Mount Wilson Observatory in California – the largest telescope in the world between 1917 and 1949 – but even it was not large enough to resolve the disk of Betelgeuse. Instead Michelson turned the telescope into an interferometer by attaching a framework with 4 six inch mirrors to the front of the telescope.

Michelson interferometer for measuring star diameters, attached to front of the 2.5m Hooker telescope.

The mirrors created 4 separate light paths of the star that combined, due to the wave nature of light, into an interference pattern.

Astronomical interferometers can produce higher resolution astronomical images than any other type of telescope – theoretically producing images with the angular resolution of a huge telescope with an aperture equal to the separation between the light paths. The separation between the mirrors of Michelson and Pease’s interferometer at Mount Wilson was about 20 feet (or 6 m) and this proved sufficient for them to measure the angular diameter of Betelgeuse as 0.047 arc-seconds.

Once you know the angular diameter, then if you also know the distance of the star, you can easily get its linear diameter in space. Michelson and Pease used several estimates of the distance to Betelgeuse available in 1920 to come up with a linear diameter of 386,000,000 km, somewhat smaller than more modern estimates. Using their 0.047 arc-seconds angular diameter with current estimates of the distance to Betelgeuse (about 724 light years) gives a linear diameter of 1.83 billion km — a truly colossal diameter, the equivalent of over 1,300 solar diameters!

Michelson and Pease published their results in the May 1921 edition of the Astrophysical Journal but a summary “Betelgeuse: How its Diameter was measured” appeared one month earlier in the April 1921 edition of the RASC Journal.

Betelgeuse is a peculiar star that is subjected to pulsation cycles that not only make is brightness vary but also make its size vary. Long-term monitoring by UC Berkeley’s Infrared Spatial Interferometer (ISI) on the top of Mt. Wilson show that Betelgeuse shrunk in diameter by more than 15% from 1993 to 2009. Recent images show that Betelgeuse has an asymmetric surface and appears to be shedding gas and dust at tremendous rates.

Asymmetric surface of Betelgeuse in Jan 2019
Astronomers used ESO’s Very Large Telescope to discovered a plume of gas ejected from Betelgeuse and a gigantic bubble that boils away on its surface. 
Image Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Since Betelgeuse has a radius 1,300 times that of the Sun, it has a volume about 1.3 billion times larger than the Sun. But its mass is only about 8 – 20 times the Sun. This means the density of Betelgeuse is much, much lower than the Sun. The average density of the Sun is about 1.4 grams/cc – somewhat higher than the density of water. In contrast, the average density of Betelgeuse is just 12 billionths of a gram/cc. This is about a million times less dense than Earth’s atmosphere at sea level, or about the same as a vacuum found in an insulating Thermos bottle.

Feb. 18, 2011

Members: Leigh, Mark, Wayne, Oleg, Janeen & Clive

Arrive: 7:30 pm

Temp: 1 C

Weather clear with full moon.

Leigh opened dome and uncovered equipment in dome in anticipation of observing with LX200. The computer in the dome failed to boot. Tried several attempts with no results. Oleg gave it a try as well. We concluded that it seemed to be a problem with the hard drive. Unplugged and removed PC to take home and do further tests. Re-covered the telescope and other equipment and shut down power to dome.

Janeen brought her new Williams Optics 110mm Megrez APO. Very nice! Wayne took some photos of the Moon with his DSLR through Janeen’s new scope.

Unfortunately, Oleg had some difficulties with batteries and was unable to get his mount to work that night. He then lent his assistance to everyone else.

Clive and Janeen departed at 10:15pm

Wayne and Oleg departed at 10:30 pm

Mark and Leigh worked with Mark’s Tak and were able to get his Robo-Focus working properly. YEH! Continued working with Mark’s equipment taking photos with Mark’s DSLR.

Departed: 2:10 am

Temp: -2C

Earth Hour 2011: It's time to go beyond the hour

At 8.30 PM on Saturday 26 March 2011, lights will switch off around the globe for Earth Hour.

This year, when the lights go back on, we want you to think about what you can change in your daily life that will benefit the planet. To share your stories, thoughts and ideas with us and to get inspiration from what others are doing. So tell us what you’re going to do and we’ll tell the world.

Together our actions add up.

Visit Earth Hour Global Site at:

Earth Hour started in 2007 in Sydney, Australia when 2.2 million individuals and more than 2,000 businesses turned their lights off for one hour to take a stand against climate change. Only a year later and Earth Hour had become a global sustainability movement with more than 50 million people across 35 countries participating. Global landmarks such as the Sydney Harbour Bridge, CN Tower in Toronto, Golden Gate Bridge in San Francisco, and Rome’s Colosseum, all stood in darkness, as symbols of hope for a cause that grows more urgent by the hour.

In March 2009, hundreds of millions of people took part in the third Earth Hour. Over 4000 cities in 88 countries officially switched off to pledge their support for the planet, making Earth Hour 2009 the world’s largest global climate change initiative.

On Saturday 27 March, Earth Hour 2010 became the biggest Earth Hour ever. A record 128 countries and territories joined the global display of climate action. Iconic buildings and landmarks from Asia Pacific to Europe and Africa to the Americas switched off. People across the world from all walks of life turned off their lights and came together in celebration and contemplation of the one thing we all have in common – our planet.

Earth Hour 2011 will take place on Saturday 26 March at 8.30PM (local time). This Earth Hour we want you to go beyond the hour, so after the lights go back on think about what else you can do to make a difference. Together our actions add up.

A chance to address LPA issues with BC Hydro at an open house

Attention All LPAer’s! – Here’s our chance to shine, down, not up!

gastown clock at night

BC Hydro wants to hear from you about how we set the course for a Clean Energy Future. To add your voice, attend a public open house in a community near you. Registration is not required for these events.

Here is our chance to ask BC Hydro what they are doing about LPA issues on a province-wide scale.   Maybe even show them a photo or two!   Ask some questions.

This event will occur in Vancouver  on Tuesday, March 15, 2011 at 6:00 – 9:00 p.m. at the  Simon Fraser Morris J. Wosk Centre for Dialogue.

Also at Abbotsford Wednesday,
March 16, 20116:00 – 9:00 p.m.Clearbrook Community Centre.

For full details, please visit:

We could ask them:

– What is being done to establish & enforce lower & even lighting levels?

– When will non full cut-off lights be stopped being sold?

– When will new lights replace obsolete non full cut-off fixtures (street / road, parking, yard etc)?

– What is their stand on light trespass, skyglow, and glare reduction & elimination.

This is an opportunity for having our voice heard, power reduce waste (saving all citizens money also), and preserve the night sky.

This information was provided by:

Maurice Sluka

Royal Astronomical Society of Canada Prince George Centre

Media Relations & Past President

Planetary Society (member)

International Dark Sky Association (member)

Correct Lighting – Up Close And Personal

correct lighting - up close and personal
correct lighting - up close and personal

Look closely at the way this light is designed.  Its a seal unit with no way for light to escape from the top.  In addition, its using low wattage flourescent light blub technology.

This light is a good trade-off between long life, aesthetics and practicality.  We can all learn a few things from the designer.


Correct Outdoor Lighting

Correct Outdoor Lighting

correct outdoor lighting
correct outdoor lighting

Kudos to the lighting designer for this building.

The lighting on the Davis County (Utah) Library is a model of correct outfoor lighting techniques. Nothing glares in your eyes, no light trespasses on to adjascent properites. It’s lit to appropriate levels. I tried to make sure that the image I saw on the camera LCD matched what I saw with my eyes. The only difference was that I could actually see the stars.

It’s not the number of photons, but where they are directed that makes effective lighting.


What Is Skyglow?

skyglow as seen from earth
skyglow as seen from earth

One important issue with observing artificial night lighting from space that needs to be addressed is a phenomenon known as skyglow. Even in its pristine state the night sky is not completely dark. Some light comes from the stars, some from sunlight scattered by space dust in the plane of the solar system, and some from atmospheric gases subject to radiation and particle fluxes mostly from the sun (Clark 2008). This is called natural skyglow. Light emitted from human settlements in the atmosphere is refracted or scattered by air and water molecules and suspended particles (atmospheric aerosol) caused by dust, pollen, salt from sea spray, and waste products from industry (House of Commons 2003). Artificially illuminating the sky over great distances this is called artificial skyglow. In particular in the field of astronomy skyglow obscuring the night sky is an issue of utmost importance with extensive scientific research being conducted in recent years. Baddiley’s guide ‘Towards Understanding Skyglow’ (2007a) lists different sources contributing to skyglow in urban and rural areas. Furthermore a mathematical model of skyglow is presented considering different skyglow mechanisms (i.e. directly radiated light above the horizontal; reflected light from the road, ground and other surfaces; light scattered by air molecules; light scattered by aerosols) and different types of luminaires. According to Clark (2008) the total artificial light flux emitted by a city tends to be proportional to the product of two quantities, (1) the
number of light sources and (2) their mean output of light. Related to a growing economy and urban population growth typically both of these quantities increase over time. Considering artificial skyglow entails that the DMSP satellite sensors record much larger areas than just the immediate location of the lighting sources. Using satellite observed nighttime lights for delineating urban areas (Small et al. 2005) and approximating impervious surfaces (Elvidge et al. 2007) requires eliminating skyglow from the data, i.e. by applying thresholds to the digital number values. When dealing with ecological issues skyglow is a significant factor of light pollution as already very low light intensities alter the natural environment. Following recent approaches of modeling ecological impact of artificial night lighting (Aubrecht et al. 2008a) for the present analysis skyglow is thus not modeled out but rather considered as important contribution.