Feb 2018 – ‘black’ moon … and Betelgeuse the friendly red (super)giant

by Pat Browne

Now that we’re over the media event of the Jan 31st 2018 Blue, Blood, Super… moon…

  • ‘Blue’, (2nd Full moon in the month)
  • ‘Blood’, (Reddened due to partial or full lunar eclipse)
  • ‘Super’ (On or near perigee (closest point to earth in lunar orbit)).
Partial Lunar Eclipse viewed outside of Mississippi Mills – early morning Jan 31 2018 Image Courtesy Stephen J. McIntyre

…let’s consider what can be seen in the ‘absence of the full moon’ this February:

Month without full moon can be called a ‘black’ moon because we get new moon (dark) in February and just barely miss the full moon date.

This can occur only in February; it happens about every 20 years. When February is without full moon, then the preceding January or December and the following March or April have two full moons.[1]

As with the case of two new moons in a month, whether a black moon by this definition occurs depends on the time zone. Calculated in UTC, instances of a month without full moon between 1990 and 2040 are:courtesy wikipedia Black Moon

  • February 1999
  • February 2018
  • February 2037

Note that the term ‘black’ moon is just as fanciful as ‘blue’ or ‘blood’. It does not mean that the complete lunar cycle will  not occur. It simply means that the calendar date for Full Moon falls outside of the month of February in 2018.

What about actual  ‘colors’ or ‘spectra’ of Stars – like Betelgeuse, the red (super) giant

Red supergiants are stars with a supergiant luminosity class  of spectral type M or K. [ Betelgeuse has spectral type M1b].  Supergiants are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

– courtesy https://en.wikipedia.org/wiki/Red_supergiant_star


Spectral Classification according to Temperature: Betelgeuse is spectral type M – very cool!

Although red supergiants are much cooler than the Sun, they are so much larger that they are highly luminous, typically tens or hundreds of thousands L [luminousity of the sun] There is an upper limit to the luminosity of a red supergiant at around half a million L. Stars above this luminosity would be too unstable and simply don’t form.

In the night sky, Betelgeuse is easy to spot with the naked eye owing to its distinctive orange-red color. In the Northern Hemisphere beginning in January of each year, it can be seen rising in the east just after sunset.

Betelgeuse in Orion – Feb 2 (early evening), courtesy Stellarium
Stellar portrait in Orion – featuring Betelgeuse Courtesy Sky and Telescope

Betelgeuse – Red (Super)Giant Type M star

  • Ninth brightest star in the night sky and second-brightest in the constellation of Orion. The star is classified as a red supergiant and is one of the largest and most luminous stars visible to the naked eye. If Betelgeuse were at the center of the solar system, its surface would extend past the asteroid belt, wholly engulfing the orbits of Mercury, Venus, Earth and Mars. Calculations of its mass range from slightly under ten to a little over twenty times that of the Sun

Giant Diameter, Low Surface Temperature – Redder appearance:

Because the energy is spread across a larger area, surface temperatures are actually cooler, reaching only 2,200 to 3,200 degrees Celsius (4,000 to 5,800 degrees Fahrenheit), a little over half as hot as the sun. This temperature change causes stars to shine in the redder part of the spectrum, leading to the name red giant, though they are often more orangish in appearance.


courtesy http://www.astronomycameras.com/blog/archive/20100210/dmk-images-with-star-analyser/

The above image of spectra taken with a diffraction grating shows the maximum intensity under the curve residing in the red part of the spectrum. Radiation reaching our eyes on earth is dominated by longer wavelengths producing that nice orange tinge. The spectral hues of Betelgeuse tell us something real about the the physical processes at work. What’s going on with this red supergiant…?

We’ve got furious nuclear fusion at work Hydrogen to Helium and now Helium to Carbon and Oxygen…  Take 10 minutes to watch Sal explain…

Becoming a red giant | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy

So subtle, but colorful hues from stars reveal the physical process of nuclear fusion. To get some guide on how to observe these stellar delights in February  – see Millstone Night Sky – Giant, Luminous Winter Stars