A pdf version of this member’s newsletter which includes an additional section on EAS business and developments is issued to all current members.
See also our monthly Sky Notes for objects observable in the sky from Kendal during this month.
Barnard’s Star – Graham Fell
I thought I would write a piece on Barnard’s Star because I have heard of it but have absolutely no idea what it is. So, I’m sitting down at Wikipedia to learn about it and one of the joys of old age is learning something new. Where would we be if we knew it all? I apologise if you (the reader) already know everything about Barnard’s Star because with a newsletter you can just move on down to the next piece – unlike a real live lecture!
Now I know it is the 4th nearest known star after the three parts of the Alpha Centauri system and it is the closest star in the northern celestial hemisphere. It is a Red Dwarf which reminds me that during the lockdown, many episodes of another Red Dwarf were shown on TV so I could relive that great series from long ago. I even liked the new ones they did a couple of years back.
But – enough of this trivia – Barnard’s star is a red dwarf with a mass of only 1/7th that of our sun. Now that surprised me as I thought a red dwarf would be much more massive. Maybe one of our members could do a piece on Red Dwarfs and so explain to me why this star is so small! So, Barnard’s Star is not visible to the naked eye as it has a magnitude of about +9.5 but it is brighter in the infrared than it is in visible light.
This is a map of where it is and thanks to Wikipedia for the image on the left and Universe Today for the virtual sky image on the right.
It was named after an American astronomer in 1916 as he measured its proper motion of 10.3 arcsecs per year relative to the Sun and this is the highest known for any star. Here I’ve taken an animated GIF from Universe Today’s website showing its apparent movement. The movement is from 1985 to 2005 – so a 20- year period.
The apparent motion of Barnard’s Star against the background stars due to its proper motion. The images are roughly 5 years apart.
I am quite amazed to see a star move so much and one that is quite so close to us. It could be that its movement is due to the effect of Dave Lister having curries for breakfast! Dave Lister was only 3 million years old but Barnard’s Star is some 7 to 12 billion years old and so it could be one of the oldest stars in the Milky Way. It has lost some of its rotational energy but its brightness changes indicate a rotational period of 130 days compared to the 25 days of our Sun. With such a long age it was assumed that it would be quiescent in terms of stellar activity but in 1998 astronomers observed an intense stellar flare showing it to be a flare star.
One of the things that always amazes me is that in Astronomy, things are sometimes very different to what we expect. The whole history of astronomy is a universe of surprises every time we learn something new. (Look at what New Horizons found about Pluto!)
This star has roughly 150 times the mass of Jupiter but its radius is only 1.5 to 2 times larger, due to its much higher density. So, Jupiter is a tiny wispy cloud compared to Barnard’s star whereas I always imagine Jupiter as a massive planet thankfully picking up a lot of debris over the last 4 billion years and so saving our planet from death and destruction.
However, Barnard’s star gave us unexpected news in 2018 when an international team of astronomers announced the detection of a super Earth orbiting quite close to the star. Wikipedia tells us that ” was found near the stellar system’s snow line, which is an ideal spot for the icy accretion of proto-planetary the planet material” and this super planet orbits at 0.4AU every 233 days and has a mass of just over three Earths. With an estimated surface temperature of -170°C, it is not likely to be on Dave Lister’s list of planets to visit!
So, after this brief precis of the facts about Barnard’s Star, for which I thank Wikipedia, I can now bore people with all this information.
I am now looking forward to learning so much more – hopefully from members like me who decide to find out some information on one topic and share it with the Society. This short piece alone opens up topics like flare stars, red dwarfs, stars that move, snow lines etc.
New Horizons delivers again – Ian Bradley
You will know of the stunning results from the New Horizons probe from its passage by Pluto and the Kuiper Belt object Arrokoth. Well, it’s just got better… OK, I might be over-egging this!
You are probably aware that the distance to nearby stars can be measured using parallax. That is, the position of a nearby star appears to change relative to distance stars as the Earth orbits around the Sun, with it returning to its original position after a year. This is different to proper motion, in Graham’s article above, that is caused by the actual motion of the star so that the star never returns to than earlier position. Measuring this angular change, the parallax angle, from opposite sides of the Earth’s orbit gives, by simple trigonometry, the distance to the star as we know the Earth-Sun distance, 1 AU ≈150 million kilometres.
The problem is that these angular changes are tiny. The nearest star system to us is the Alpha-Centauri system (α, β and Proxima) is about 4.2 light-years away. For α-Centauri, the angle is only 0.74 arc seconds [symbol “], where 1″ is 1/3600 of a degree. For comparison, Jupiter is currently 45″ and the Moon 1800″ in diameter. The seeing due to the Earth’s atmosphere at the best observing sites on Earth is around 0.1″ and in Kendal is typically 1.5–2”, so you can see this is a formidable challenge requiring sophisticated image analysis. Hence the method only works out to a few hundred light-years for ground-based telescopes. However, being above the atmosphere, the Gaia satellite has measured stars out to 20,000 light-years from near-Earth orbit.
Click to enlarge
Proxima Centauri as simultaneously seen from the Earth and the New Horizons probe. The apparent position change against the more distant background stars is clear. Credit: New Horizons Collaboration
So, what’s new? New Horizons imaged two stars, Proxima Centauri, 4.2 light-years away, and Wolf 359, 7.9 light-years away, at the same time as they were imaged from Earth. The change in position of each star is really clear due to the different viewing perspectives – see the animated gif on the EAS website – and note that the change for Proxima Centauri is larger than that for Wolf 359, as Proxima Centauri is nearer to the Earth.
What’s new here is that New Horizons is 46 AU away from the Earth giving a much longer baseline and hence a larger apparent positional change of the stars against the background of very stars. Whilst this isn’t ground-breaking, and it is unlikely we’ll ever position a Gaia type survey satellite that far out in the near future, I still find this interesting.
BepiColumbo mission to Mercury – Graham Fell
Mercury has not been a major target for NASA, possibly because it is so close to the Sun and the fact that it spins so slowly on its axis. Its surface temperature varies from 427°C (which is twice as hot as your oven) to -173°C, which in our house is on the ‘nippy’ side.
As Mercury travels so fast around the Sun (24 to 30 miles per sec) a spacecraft has to be travelling pretty fast to get into an orbit. The Sun’s gravitational pull doesn’t help any deceleration needed to get into an orbit. Then Mercury has no atmosphere so aerobraking is of no use for any lander. So, more fuel is needed to land.
We’ve been there twice – Mariner 10 in 1974 and Messenger in 2012.
Now a third mission is on its way called BepiColumbo and it has two separate spacecraft:
- The Mercury Planetary Orbiter (MPO) which will get images in several wavelengths to map the surface and exosphere composition. MPO provided by ESA.
- The Mercury Magnetospheric Orbiter (MMO) will obviously study the magnetosphere and is provided by the Japan Aerospace Exploration Agency (JAXA)
Artist’s depiction of the BepiColombo mission, with the Mercury Planetary Orbiter (left) and Mercury Magnetospheric Orbiter (right) Source: Wikipedia
Hopefully, we will get answers to the following questions:
- What can we learn from Mercury about the composition of the solar nebula and the formation of the planetary system?
- Why is Mercury’s normalized density markedly higher than that of all other terrestrial planets, as well as the Moon?
- Is the core of Mercury liquid or solid?
- Is Mercury tectonically active today?
- Why does such a small planet possess an intrinsic magnetic field, while Venus, Mars, and the Moon do not have any?
- Why do spectroscopic observations not reveal the presence of any iron, while this element is supposedly the major constituent of Mercury?
- Do the permanently shadowed craters of the polar regions contain sulphur or water ice?
- What are the production mechanisms of the exosphere?
- In the absence of any ionosphere, how does the magnetic field interact with the solar wind?
- Is Mercury’s magnetised environment characterized by features reminiscent of the aurorae, radiation belts and magnetospheric substorms observed on Earth?
- Since the advance of Mercury’s perihelion was explained in terms of space-time curvature, can we take advantage of the proximity of the Sun to test general relativity with improved accuracy?
The mission was successfully launched on 20th October 2018 and is scheduled to enter orbit around Mercury in December 2025 and its primary mission will last for 18 months with a possible extension for one year.
The ESA-JAXA BepiColombo mission to Mercury lifts off from Europe’s Spaceport in Kourou. Credit: ESA
Finally, the Russians are proposing to launch a mission in 2031 to land on Mercury and it is called Mercury P.
The constellation of the month – Moira Greenhalgh
High up in the early September sky, slightly west of south, you will see the well-known Summer Triangle of stars, Deneb in the constellation Cygnus, Vega in Lyra, and Altair in Aquila
Let us concentrate on Lyra, only a small constellation but worth learning.
The IAU has around 74 stars in the constellation Lyra, but there are six that are the clearest, a triangle with Vega at one corner, sitting on a parallelogram. Can you have corners in a triangle?
The mythology around the constellation refers to the lyre of Orpheus, The inspiration for lots of music. Did you ever sing the Vaughan Williams’ “Orpheus with his lute made the trees, and the mountain tops that freeze, bow themselves when he did sing”? Then there is the Gluck opera Orpheo and Euridice. Who could forget hearing Kathleen Ferrier singing the aria “What is life to me without thee, what is left if thou art dead”? I suspect I may be showing my age here.
Anyway, the story is that Euridice, wife to Orpheus the great musician, was bitten by a snake, died and was taken down into the underworld. Orpheus, distraught, followed her below, charmed Hades with his singing and was allowed to bring her back to the living world provided he led her out and didn’t look back until they were both outside. When he stepped from the underworld he did look, only she was still inside. Hades took her back and it was goodbye to Euridice. He went off on other adventures such as with Jason and the Argonauts where he saved them from the Sirens, but never looked at another woman apparently.
The constellation starting point is the star Vega (α Lyrae), the second brightest in the Northern sky after Arcturus (Sirius being in the Southern hemisphere) and, at this time of the year, it is the first star to pop into view as the sky darkens. Around 12000BCE Vega was the pole star, and will be again around 14000CE, give or take a couple of days. It is a main sequence star like the sun, and was one of the first stars to be photographed, and have its spectrum recorded. It even has a debris disk around it similar to our Kuiper Belt. Sheliak (β Lyrae, bottom right of the parallelogram) is a binary system with the material being transferred between the stars. Beta Lyrae type variables are named after this star. Sulafat (γ Lyrae) is a blue giant and the second brightest star in the constellation.
Forming the small triangle with Vega are ζ Lyrae, a wide binary, and ε Lyrae also a wide binary, said to be naked eye separable. Good luck with that! Both components of epsilon are themselves binaries, hence the name “double double”. A small fifth star has also recently been found, so I don’t know what that makes the system “double double single” or “double treble”. Sounds like crochet stitches to me.
The final star of the parallelogram is δ Lyrae, an optical double and part of the sparse faint delta Lyrae cluster. Outwith the parallelogram are κ and θ Lyrae, a red giant and orange giant. Very faint are many other doubles and variables of different colours.
Lyra has many many deep sky objects, of which M57 and M56 are the most well known. M57, the ring nebula, a planetary nebula, looks wonderful in a professional telescope. For an amateur it looks like this below, not coloured but definitely a ring.
Credit: Hubble Heritage Team
The constellation Lyra was within the Kepler mission’s field of view, and many exoplanets have been found, including the 5 planets orbiting Kepler-62 of which at least 2 are within the habitable zone and are likely rocky earth like planets.
Astronomy News – David Glass
Betelgeuse – Still a Puzzle!
Ok, in our last newsletter we reported on a study using the ALMA telescope which indicated giant sunspots as the cause of Betelgeuse’s dimming earlier this year. If only that was the last word – but it isn’t. A new study using the Hubble Space Telescope is pointing towards a different cause, namely the ejection of a giant ball of hot plasma from the surface of the star. Betelgeuse got brighter at ultraviolet wavelengths during this time. This ball of plasma cooled, and dust condensed within it which blocked the light from the star and caused the observed dimming. Observations at ultraviolet wavelengths over time indicated that the plasma was ejected late in 2019 at quite a speed (200,000 mph), and the dimming started about a month afterwards. Here’s a preprint of the paper describing this, which is now published in the Astrophysical Journal.
Artist’s impression (ultraviolet wavelengths) of what might have happened to Betelgeuse late last year. First two images: a blob of hot plasma is emitted from a giant convection cell. Third image: Dust condenses out of the blob as it cools. Fourth image: view of the dusty cloud as seen from Earth.
All images Credit: NASA, ESA, and E. Wheatley (STScI).
And just as things looked like they’ve returned to normal, Betelgeuse appears to be dimming again. Photometry obtained from STEREO-A (Solar Terrestrial Relations Observatory spacecraft), at a time when Betelgeuse can’t be observed from the ground, showed a drop in V-band brightness of about 0.5 between mid-May and mid-July this year. The extent of the dimming is similar to a periodic dimming that happens, but is earlier than expected. You can read the bulletin announcing these findings here.
V-band magnitudes for Betelgeuse over time. The green and blue points are from ground-based telescopes. The red points are from the STEREO spacecraft. The gaps in the ground-based data are times when Betelgeuse is not visible.
Looking at the red points on the right, Betelgeuse is fainter now than it was in mid-April (Dupree et al.)
Following on from the successful “hop” of Starship SN5, Spacex put the next one (imaginatively called SN6) into position at Boca Chica, Texas, fitted a Raptor engine to it and lit it on 23/8/20.
It is possible that SN6 will do a “hop” to about 500 ft as early as the end of August, so keep an eye out for footage! SpaceX is evolving the rocket in small steps. SN7 could be pressure-tested until it bursts and SN8 could have three raptor engines fitted to achieve an altitude of 12 miles. However things proceed, we can expect to see dramatic tests and higher “hops” with more engines in the near future!
(Tiny) Asteroid heading for Earth
Hopefully, you saw the word “tiny” and aren’t digging your underground bunker and stocking it with bottled water and spam to last 7 years. To coincide with the presidential election in the USA on November 2nd, a wee asteroid (2018 VP1) could intercept the Earth. Or not. There aren’t enough observations yet to be certain, and the odds are about 0.4% that it will hit the Earth. This tiddler was first picked up by the Zwicky Transient Facility at the Palomar Observatory (Southern California). It’s an Apollo asteroid, which has an elliptical orbit which crosses Earth’s and an orbital period of about 2 years. It is thought to be about 2m across. The asteroid that put in a spectacular performance seen from Chelyabinsk in 2013 was thought to be about 20m across, so 2018 VP1 isn’t a cause for concern. But it could make a nice trail as it burns up, so let’s keep an eye on forecasts just in case there’s a slim chance that we can catch it!
The observations and the orbital parameters used to estimate the asteroid’s position can be found here – click on the Orbit Diagram to see how the asteroid could behave.
I was going to include an image of an asteroid hitting the Earth, but I don’t want to start anyone off digging bunkers! Oh, go on then……
Giant halo around the Andromeda Galaxy
It’s been known for a good while that late-type galaxies (star-forming, with spiral structure) contain interstellar medium (ISM), i.e. gas and dust between the stars. This is the raw material for the formation of new stars (and planets!), and is also the exhaust from old stars as they evolve and die. However, the ISM doesn’t just stay in the galaxy. It can get ejected (e.g. by supernovae) and fall back in later, extending the timescale over which new stars are formed. A recent study looked at this circum-galactic medium (CGM) around the Andromeda galaxy (M31) using the Hubble Space Telescope. The CGM is so rarefied that it hardly emits anything that can be detected, but it can obscure light from bright objects behind it – in this case, the ultraviolet light from quasars. The study found something amazing – the cloud of CGM around M31 is huge, extending over 1.3 million light-years from the galaxy centre. This is about halfway to our own Milky Way galaxy. If we could see it, it would have a greater diameter than the Plough in Ursa Major. The study also found structure within the CGM that could indicate how it formed and is evolving. You can read the preprint of the paper (now published in the Astrophysical Journal).
If we could see the circumgalactic medium around M31, here’s how it might look. (Credits: NASA, ESA, J. DePasquale and E. Wheatley (STScI), and Z. Levay (background image)
Our own Milky Way probably has a similar CGM halo. We know that M31 and the Milky Way are heading for each other and will collide in billions of years hence – however, if both galaxies have massive CGM haloes then perhaps they are starting to interact already!