March 2021

Planets to look for

Mars cruises underneath the distinctive shape of Taurus the Bull during March. Still in our northwestern evening sky, you can use the Moon to help you locate it on the evening of the 19^th of March when it will be to the left of the red planet. Above will be the V-shape of the face of Taurus the Bull, a cluster of stars known as the Hyades. Don’t mistake Mars for Aldebaran, the bright orange star on the top arm of the V.

The evening sky on the 19^th of March has the Moon to the left of Mars.

Saturn is on it’s own now, rising about 45 minutes before brighter Jupiter. Jupiter starts the month rising with Mercury, and the two planets are only a moon-width apart on the morning of the 5^th. After that Mercury gradually sinks back down towards the horizon, rising later each morning, but will be easily visible all though March. On the morning of the 11^th will be the best to catch the waning crescent Moon to the right of Jupiter and Mercury a short way below.

Venus is now hidden in the Sun’s glare and won’t be visible again until late May, when it will return to the evening sky for winter.

Constellation of the month

Vela the Sails

In ancient times there was a huge constellation known as Argo Navis, which represented the Greek ship Argo, made famous from the Greek myth of Jason and the Argonauts who went to fetch the Golden Fleece. To make things a little easier a French astronomer called Nicolas-Louis de Lacaille split the ancient constellation into four smaller ones: Carina the Keel, Vela the Sails, Puppis the Stern and Pyxis the Compass. (Actually, there was an attempt to make one part Malus the Mast but it wasn’t very popular and soon forgotten.)  At this time of year (southern hemisphere autumn) Vela can be found high in the south as it rotates above the South Celestial Pole.

Two stars in Vela make up one half of what is known as the False Cross (see the map below). People unfamiliar with the night sky often confuse this with the Southern Cross. To help tell the two apart there are a number of features to look for: The Southern Cross has a more symmetric shape (the False Cross looks a bit lop-sided), a sequence of bright to faint stars as you move around the outside (The False Cross stars are all roughly the same brightness) and the Pointers, alpha and beta Centauri, point the way to the right place! (The False Cross has no pointer stars).

IC 2602 or the Southern Pleiades

If you are in a dark location and have keen eyes you may notice a small cluster of stars surrounding the star theta Carinae. Usually referred to by its common catalogue number of IC 2602, Australians have taken to calling it the Southern Pleiades. A few people also refer to it as the “Five of Diamonds” – can you see why?

This area of the sky has the star clouds of the Milky Way running through it and a quick sweep of the area with binoculars will reveal many other clusters, large and small, nearby.

This cluster lies about 500 light years away.

The Woman behind the H-R Diagram

The Hertzsprung-Russell Diagram is one of the most important tools for astronomers. It plots the luminosity of a star against its temperature, and when we do so we find that most stars clump together into a long snaking line we call the Main Sequence, which basically describes the life of a star. The red giant branch splits off to the right half-way down, and a small but significant arc of white dwarfs curves across at the lower left. This diagram can help us determine the age of star clusters and predict what the fate of stars we see in the present time will be.

The Hertzsprung-Russell Diagram plots the luminosity of stars against their spectral class or temperature (both work) Credit: Richard Powell

Across the top of this diagram we can see the spectral sequence: O, B, A, F, G, K, M. This was arranged by Annie Jump Cannon. But this isn’t her story here, but the story of Antonia Maury, who had her birthday on 21^st March 1866. Annie and Antonia were work colleagues, they were part of a group of skilled women who worked for Edward Pickering to compile what is known as the Henry Draper Catalogue. The women were to compile the characteristics of the spectra of 225,300 stars initially for the work. It is no wonder, then, that after looking at thousands and thousands of stars that they would begin to see patterns and relationships within the spectra, and natural groupings.

Antonia Maury is recognised as being the first person to find a spectroscopic binary – that is a double star that can’t be split into two stars visually by a telescope, but the spectral lines tell us that there must be two stars orbiting one another there. She also came up with her own spectral system that differed from Annie Jump Cannon’s, and the men of the observatory who oversaw her work disagreed with her observations and refused to use it.

However, she requested that she be recognised for her work and it was published under her name, a first for women at the time, and a few years later Ejnar Hertzsprung examined her data further and found it pointed to some important relationships between brightness and distance in stars… and so we have a chart remembering the two men who publicised it and the woman who actually did the initial hard work behind it forgotten, even though it was only 120 years ago.

There is so much we owe to all the women who worked under Edward Pickering to create the Draper Catalogue. But this March let’s give Antonia Maury some time in the spotlight as her 155^th birthday comes around.