Stargazing at the End of Night

Sunset over Glen Docherty

This was me on the road and heading into the western Highlands last Saturday for my final stargazing gig of the season with the Woodland Trust.  

Skies this far north will shortly be too bright to stargaze with only Astronomical Twilight levels of darkness left near midnight and no official ‘night’ again until mid to late August. So do get out while you still can.  Of course the further south you are the less impacted you will be by this ‘near’ midnight Sun.

I had an eventful stargazing session with the Woodland Trust who were based at the Torridon for several nights. We first headed outside at about 10.30pm to view the crescent Moon with binoculars during early twilight skies – still too bright to see many stars apart from brilliant Arcturus.

The spring star Arcturus ‘Bear Watcher’ was bright enough to see in early twilight during our Moon gazing session.

After heading back inside for more projector based astronomy we ventured outside once more after 11pm and were fortunate to see a decent collection of bright stars and constellations despite some hazy cloud overhead.

Vega, Capella, Arcturus and Spica were all visible, in addition to the main stars in the Plough. I’d like to thank the Woodland Trust for inviting me and wish them well in their rewilding endeavours across the Highlands.

Earendel – The Most Distant Star Detected

The star Earendel is located at the point of the arrow in the image above, surrounded by the light from diffuse and distant galaxies.

The NASA Hubble space telescope has imaged the most distant star ever detected at a staggering 12.9 billion light years away. The light captured from Earendel (dubbed the ‘Morning Star’) is a snapshot from an epoch when the universe was only 1 billion years old, making it significantly older than the previous furthest star detected by Hubble in 2018. (That star was dated to 4 billion year after the big bang).

Normally stars at such immense distances would be undetectable, but its discovery was aided by the gravitational distortion from distant galaxy clusters, magnifying the star and its host galaxy in a phenomena called ‘gravitational lensing.

An example of a distant galaxy, revealed by the distortion of space (and therefore light) near a closer area of high mass, in the form of a galaxy cluster.

Gravitational lensing is analogous to the refraction of light from a glass lens, magnifying and revealing objects that would normally be occulted by closer structures by the bending of space near areas of high mass – like galaxy clusters. Sometimes duplicate images of the same object can be seen, creating copies of the object along symmetrical arcs. The image below illustrates this effect on a star cluster which appears either side of Earendel.

A closer image of Earendel with a mirrored image of a nearby star cluster created by gravitational lensing

You might wonder how immense distances like this can be calculated given the complexity and uncertainty in pin pointing the distance to relatively close stars, let alone objects billions of light years away?

The principle tool used to measure these vast distances is an object’s spectral redshift – a measure of how much its light rays have been stretched (made longer) due to the fabric of space itself being stretched the further away we observe. Larger redshifts indicate objects that are further away – a relationship first accurately established by Edwin Hubble when cataloging the spectra from many distant galaxies.

A measured spectra shifted towards the red end of the spectra, signalling longer wavelengths and higher recessional speeds.

Given the redshift of an object we can calculate its recessional speed (related to the global expansion of the universe) and from this its distance can be determined using the Hubble’s constant Ho. These calculations can be set out very simply:

V (recessional speed) = Red-shift x Speed of Light

In the case of Earendel the detected redshift from its spectra was 6.2. Therefore:

V (Earendel) = 6.2 x 300 million m/s = 1860 million m/s.

It’s important to note that this speed is faster than the speed of light! How can this be? Well this is actually a measure of the speed that space itself is expanding. Light cannot travel faster than 300 million m/s – our cosmological speed limit – but there is no limit on the rate at which the fabric of space can expand. In fact for general relativity to work space must be permitted to expand at potentially unlimited rates.

From the recessional speed we then use Hubble’s law to find the distance to the star:

D (distance) = V (recessional velocity) / H0 (Hubble’s constant)

This gives our published distance to Earendel of 12.9 billion light years! A staggering distance taking us back to the earliest period of star formation when the abundance of atomic elements in the universe was very different to today.

We believe the very first population of stars emerged around 100 to 250 million years after the big bang, so Earendel formed only a few hundred million yeas after this. The new James Webb telescope will likely continue to study Earendel in the infrared, at longer wavelengths, potentially revealing the star’s temperature and luminosity and therefore its stellar classification.

The Physics of Visible Star Light

Globular cluster NGC 6717. The colour of light from the stars is a general indicator of surface temperature.

The colour of a star tells us how hot it burns. From the dull red of Arcturus to the brilliant blue of Rigel, you can actually see these subtle colour differences with your own eyes when looking up at the night sky.

Just like an iron cast into the blacksmith’s forge, which slowly changes from red to white hot, stars emit light at different frequencies depending on their overall luminosity and energy output.

The same relationship between colour and temperature is noted when metal objects are heated.

The Planck-Einstein equation E = hf is a basic way of understanding this. E is energy, f is frequency and h is the famous Planck’s constant. Higher frequency light (blue) is more energetic than lower frequency light (red) and therefore hotter and more luminous stars tend to appear more blue. Meanwhile cooler stars whose external atmospheric envelopes has expanded (red giants like Betelgeuse) appear redder.

A simple way to highlight the colour of star light is to take your smartphone camera or DSLR and manually defocus it on a target star. This will emphasise the colour and you can even produce beautiful star trails like the one below by taking a movie or long exposure star trail.

Star trail image credit – Amanda Cross

Tales of Dark Matter

All the light we see from distant stars and galaxies is made from visible matter, yet evidence from the rotational speeds of other galaxies suggests dark matter outweighs visible matter on a ratio six to one. Image: ‘Our galaxy Over Achnasheen’, Stephen Mackintosh

Join me up at Abriachan Forest (a Dark Sky Discovery site) for an evening of stargazing and astronomy on February 25th with our first guest speaker of the 2022 season – Professor Martin Hendry.

If skies are clear Martin and myself will host an outdoor stargazing session, discussion and Q&A under the stars. Following this Martin will present his indoor guest talk on the very latest discoveries in cosmology, concentrating on the elusive nature of dark matter and dark energy.

Refreshments provided plus binoculars for stargazing. Under 16s with accompanying adults go free. Tickets can be booked via Eventbrite here or you can reserve directly from my Facebook page here.

Martin speaking at the Science on Stage Festival

Martin Hendry is Professor of Gravitational Astrophysics and Cosmology at the University of Glasgow and is a passionate advocate for STEM education and science engagement with schools and public audiences. He is the author of more than 200 scientific articles and is a senior member of the LIGO Scientific Collaboration, the global team of more than 1400 scientists which made the first-ever detection of gravitational waves – a discovery awarded the 2017 Nobel Prize for Physics. Martin is a Fellow of the Institute of Physics and the Royal Society of Edinburgh and is currently a Trustee of the IOP and the James Clerk Maxwell Foundation. In 2015 he was awarded an MBE for services to the public understanding of science.

Dark Sky Burns 2022

Starry skies over Abriachan Forest

On Saturday the 29th January, join me up at Abriachan Forest (a Dark Sky Discovery site) for an evening of stargazing and astronomy with a Burns night twist

If conditions are clear I’ll be guiding you under the Milky Way class dark skies of Abriachan Forest (with a backup astronomy presentation if clouds roll in).

Meanwhile the Abriachan team will host an outdoor Tam’s Trail to find signs of Meg and a cutty sark! Plus Haggis hand warmers and refreshments for a simple Burn’s supper fare.

Booking in advance via Eventbrite is essential due to site capacity. Ticket links here.

2022 Hebridean Dark Skies Dates

Please see confirmed dates for my stargazing tours next February for the 2022 Hebridean Dark Skies Festival. I hope to be hosting a walk and talk under the stars from each location with an indoor fallback in the event of poor weather (so please book with confidence). Ticket links below:

“Astronomer Stephen Mackintosh will host stargazing events at Calanais Standing Stones and Visitor CentreScaladale Centre and Grinneabhat (Bragar and Arnol Community Trust) as part of the 2022 Hebridean Dark Skies Festival. #hebdarkskies

Tickets are on sale now – book here:
Calanais, 17 Feb: https://lanntair.com/…/highland-astronomy-at-calanais/
Grinneabhat, 18 Feb: https://lanntair.com/…/highland-astronomy-at-grinneabhat/
Scaladale, 19 Feb: https://lanntair.com/…/highland-astronomy-at-scaladale…/

The Hebridean Dark Skies Festival runs from 11-25 February. Look out for lots more programme announcements in the next few days – and for our printed festival programme, available at An Lanntair and across the island from next week! Full listings at https://lanntair.com/events/category/dark-skies/

#hebdarkskies is supported by CalMac FerriesHighlands and Islands Enterprise (HIE) and Culture Business Fund Scotland, in partnership with Lews Castle College UHIGallan Head Community TrustCalanais Standing Stones and Visitor Centre and Stornoway Astronomical Society.”