Our Supermassive Black Hole – Sagittarius A

Sagittarius A – as imaged by the team at the Event Horizon Telescope.

Imagine taking over 4 million copies of our Sun and cramming the combined mass into a region of space no bigger than the orbit of Mercury.

That’s Sagittarius A, the supermassive black hole at the centre of our own Milky Way galaxy. Evidence for Sagittarius A has been growing since the 1970s but now in 2022 the team at the Event Horizon Telescope have actually imaged it.

The term ‘supermassive’ when attributed to black holes is very misleading as black holes are incredibly low volume but dense regions of space. To give you a feel for this, if you took our Moon and somehow compressed it into a black hole, the resulting anomaly would have a diameter of 0.2 millimetres! That’s probably less than the size of a single pixel on the screen you’re reading this article on.

As black holes grow they can devour more mass and will slowly get bigger with the event horizon radius r defined by the famous Schwarzschild equation:

r = 2GM/c^2

In this equation G is the universal gravitational constant, c is the speed of light and M is the mass of the black hole. This is a simple linear relationship, so for example doubling the mass of a black hole will double its radius.

The bright central region of our Milky Way galaxy where Sagittarius A is located. Telescopes of the Atacama Large Millimeter/submillimeter Array in the foreground. Credit: EHT Collaboration

Given the relatively small volumes and areas of space involved, detecting even the most massive black holes in challenging to say the least. Sagittarius A, despite containing millions of solar masses, occupies a volume smaller than a single star in its giant phase of evolution. This gets compounded by the incredible distances involved. The centre of our Milky Way where Sagittarius A is located is a staggering 26,000 light years away. How then did the team capture the image?

The key was using multiple detectors spread across the planet, effectively constructing an Earth sized telescope. The data collected from these widely spaced arrays was then gathered together, producing many terabytes of data, and processed by banks of supercomputers called ‘correlators’. The final image was constructed using advanced algorithmic and statistical imaging techniques.

EVT’s detectors are spread around the planet effectively creating an Earth sized detection aperture.

Clearly by their nature black holes do not allow any light to escape so what we see in the final image is the infrared signature of super-heated gas rotating close to the black hole.  Black holes therefore reveal themselves by their indirect influence on nearby objects rather than direct observation.

Indeed, Sagittarius A’s existence was originally inferred by its influence on nearby stars, which are being thrown about at fantastically high speeds due to its intense gravitational influence. Fast enough for us to produce time lapse images over several years (see animation below).

Time-lapse from the ESO’s Very Large Telescope in Chile shows stars orbiting close to Sagittarius A over a 20 year detection period. 

Donated 200mm Telescope for Abriachan

Fully restored Sky-Watcher 200mm telescope for Abriachan Forest

I recently restored a lovely 200mm reflector, kindly donated to our astronomy programme at Abriachan Forest. This one came from @BigSkyLodges over in the Black Isle. Many thanks to Martin Hind for the heads up.

Just needed a clean and some TLC and now back in good working order. This adds to our 150mm reflector and will compliment our binocular stargazing events when we kick off next season.  

A 200mm like this is ideal for observing fainter deep sky objects like galaxies, planetary nebulae and globular clusters. 

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