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Mars, the only bright planet currently in the evening sky, can be seen passing between two prominent star clusters during March, 2021. They are the Pleiades and the Hyades, in the constellation of Taurus.
Both star clusters are distinctive and quite easy to find in the night sky, even when they don’t have a bright planet close by! The Pleiades are the more compact group, popularly known as the Seven Sisters, and also catalogued as Messier 45, of M45. The cluster formed in the last 100 million years, and its stars lie at an average distance of about 444 light-years from us.
Long-exposure photos show the Pleiades to be enveloped in gas. It used to be thought that this was gas left over from the stars’ formation, but we now know that it is a cloud of gas that the stars just happen to be passing through and illuminating.
There are hundreds of stars in the Pleiades cluster, but seven was the number that were said to be visible with the unaided eye in good skies. Nowadays, most people can readily see six. Many more are brought into view with binoculars.
The Pleiades, being so conspicuous, feature in many cultures around the world. One highly descriptive reference to them comes in the poem Locksley Hall, by Alfred, Lord Tennyson, who referred to them as the Pleiads. He wrote:
“Many a night I saw the Pleiads, rising thro’ the mellow shade,
Glitter like a swarm of fire-flies tangled in a silver braid.”
The Hyades are a closer star cluster – the nearest to us in fact, at an average distance of 151 light-years – and the brightest of its hundreds of stars form a V-shape as viewed from Earth. However, the brightest star, at one end of the V, is unrelated to the cluster.
Aldebaran is a star that lies less than half the distance from us, about 65 light-years, and it just happens to be between us and the cluster, so its place in the Hyades is purely a line-of-sight effect.
The Hyades are a much older cluster than the Pleiades, with an age of around 625 million years.
Mars is receding from Earth, and currently at a distance of around 240 million km, shining at around magnitude 1. This is a lot fainter than when it was at opposition last October, just 62 million km from Earth and shining at a brilliant -2.6, but it is comparable to the brightest stars. In March, it will appear of similar brightness to Aldebaran, though slightly fainter.
The eastward motion of Mars from night to night will be quite detectable, and very obvious over just a few nights, as shown in our illustrative chart above. If you have a camera capable of taking time exposures of a few seconds, you might like to take photos on different nights to record for yourself the movement of Mars between the Pleiades and Hyades.
The waxing crescent Moon will lie close to Mars, and between the two clusters, on 19 March, adding to the spectacle. Don’t miss that!
The author was lucky to get a run of clear nights last year when another planet, Venus, passed very close to the Pleiades. By stacking the individual exposures together, he produced a photo showing the planet’s changing position from night to night.
Related: Our guide to Mars
Related: The night sky this month
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