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Seven Odd Stars

Dark matter

Dark matter: It’s illusive in every respect, outweighing the more well-understood visible matter by around six to one, making up 27% of our entire Universe.

And unlike normal matter, Dark matter cannot be seen: it doesn’t absorb, reflect or emit light, instead interacting only by its gravitational effect [1], theorised because of its effect on the mass of galaxies and galaxy clusters, like 'Abell 1656' or the 'Coma Galaxy Cluster'.

Galaxy clusters revolve, like a carousel, with the speed of this movement dependent on both the mass and position of the galaxies (mass calculated using light coming from the cluster). However, Fritz Zwicky, a Swiss Astronomer, noticed it wasn’t revolving at the right speed for its mass. Instead, it was going at a speed suggestive of a greater mass, from another, invisible, source: dark matter.

This has since been measured in many galaxy clusters, as well as galaxies, raising the question of “What are we missing?”. The Fermi Gamma-Ray Telescope (FGST for short) is one of many projects hoping to solve this. One part of its mission is to find possible gamma ray emissions due to the collision of dark matter particles [2]. It looks at dwarf spheroidal galaxies (dSphs) -galaxies with little dust, old stars and low light emission-, the galactic centre -the centre of our Milky Way, where there may be an excess of dark matter-, galaxy clusters -the largest gravitationally bound structures, filled with dark matter-, and background emissions -with dark matter particle collisions thought to have happened throughout the Universe’s history-[3].

Infrared/Visible Image of Coma Cluster

False Colour Infrared/Visible Image of Coma Cluster.

Credit: NASA/JPL-Caltech/L. Jenkins (GSFC).


Axions are illusive particles, important because they could be one part of the answer to the question of a century: “What is dark matter?”. If they are real, they’re low mass elementary particles that could make up most, or all, of what we call “dark matter”.

They’ve been found to possibly work like neutrinos [4]: discovered because of a lack of conservation of energy, momentum and angular momentum (or spin) [5], with detectors like Super-Kamiokande (or Super K) in Japan. It uses a large stainless-steel tank filled with 50,000 tons of pure water, put 1,000 metres underground, using 13,000 bubble-like photon-multipliers (converts photons, particles that make up light, into electrical signals) to detect these particles, which are in abundance [6].

So could we detect and study axions in this way? Quite possibly, since they’re likely produced in extreme environments, like star cores (like in our Sun)[7].

In fact, there is one such experiment, run at CERN (European Organisation for Nuclear Research), called the CERN Axion Solar Telescope, or CAST for short. It uses a strange type of telescope, using everything from a hollow beam pipe (like the tubes from normal telescope) to a dipole magnet (a prototype of that used in the Large Hadron Collider -LHC), along with an X-ray focusing mirror system and X-ray detectors at each end. But, why X-rays? Well, the magnetic field produced by the tube will convert axions into X-rays (read on to find out more about this process), making them easy to detect, since, as you will find out, X-rays from Space can’t be seen on Earth, even at the more high-tech observatories [15].

CAST Experiment at CERN. Credit: Roland Hagemann, CC BY-SA 3.0

The Magnificent Seven

Yes, it’s a film, but it’s also a fun name for a group of seven neutron stars (also called the X-ray Dim Isolated Neutron Stars, or XDINS for short [8]): RX J0420.0-5022, RX J0720.4-3125, RX J0806.4-4123, RBS1223, RX J1605.3+3249, RX J1856.5-3754, and last but not least RBS 1774 (catchy!) [9].

They’re important because, for their age, they emit too many ultra-high-energy X-rays [4]. All stars have a lifecycle that can be plotted on a Hertzsprung-Russell Diagram: a diagram which relates luminosity (how bright) to temperature (what colour they are, using the letters OBAFGKM to denote this, O being the hottest and bluest, whilst M is the coolest and reddest). Stars, depending on their mass, follow various patterns across the graph [10].

However, as recently found by using archive data from ESA’s XMM-Newton (or X-ray Multi-Mirror Mission) Space Telescope and NASA’s Chandra X-ray Space Telescope, these stars aren’t following suit, pretending to look older than they really are, as though they’re further along the path stars take along the diagram (although neutrons stars aren’t usually found on it, since they’re extremely hot and extremely blue). This seems pretty odd, but there may be a good explanation for this, and it lies in the illusive axion.

They’re special, not only because the may hold the key to discovering what dark matter is, but because they’re expected to be created in the core of stars, creating high-energy X-ray photons when inside a strong magnetic field: a feature of the Magnificent Seven (with magnetic fields billions of times stronger than that found on Earth).

In order to cement this theory, the next step is seen to be observing white dwarfs (remnant of a low-mass star, like the Sun) with X-ray telescopes, since they have very strong magnetic fields, but aren’t expected to produce x-rays [11].

Hertzsprung-Russell Diagram

Hertzsprung-Russell Diagram. Credit: ESO, CC BY 4.0

For Your Eyes Only

It isn’t easy though, since observing in the high-energy X-ray part of the electromagnetic spectrum (a spectrum from red low energy to blue high energy light) is impossible from Earth based telescopes, with our atmosphere being opaque to high energy X-ray emissions (which is good for our health, but less so for science -you can’t have everything!). Instead, the only method is by taking up valuable time on the few X-ray telescope orbiting Earth, like XMM-Newton [12].

The telescope, launched on 10th December 1999, uses 58 mirrors to detect millions of stars in one observation, even if they’re very dim and far away, using “five X-ray imaging cameras and spectrographs” [13].

And then there’s Chandra, launched a few months before, on 23rd July 1999, with just 4 mirrors, nested inside each other, focusing light onto detectors 9.2m from the front of the telescope [14].

Transparency of Atmosphere

Transparency of Atmosphere (X-ray = left, radio = right). Credit: ESA/Hubble (F. Granato).

What ever research into why these stars are so bring brings up, it will still revolutionise our understanding of the Universe, even if the extra emissions aren’t made by quite the process we anticipated.

by George Abraham, ADAS member.

Click here for the previous news article

Click here for the next news article

Click here to see the CAST experiment in action

Click here to see a video of stars, imaged by ESA's Gaia mission, get collated into the Hertzsprung-Russell diagram

Click on the names to have a look at what the Magnificent Seven look like with XMM-Newton data on ESA Sky: RX J0420.0-5022, RX J0720.4-3125, RX J0806.4-4123, RBS1223, RX J1605.3+3249, RX J1856.5-3754, RBS 1774



  1. "Dark matter". CERN. Archived from original on 23rd January 2021.

  2. "What is Dark Matter". NASA. Archived from the original on 23rd January 2021.

  3. "Fermi Searches for Dark Matter". Fermi Gamma-ray Space Telescope, NASA. Archived from the original on 23rd January 2021.

  4. "Mystery particle may explain extreme X-rays shooting from the 'Magnificent 7' stars". Archived from the original on 23rd January 2021.

  5. "How Massive Neutrinos Broke the Standard Model". Forbes. Archived from the original on 23rd January 2021.

  6. "Overview". Super-Kamiokande. Archived from the original on 23rd January 2021.

  7. "Search for axions from nearby star Betelgeuse comes up empty". Archived from the original on 23rd January 2021.

  8. "X-ray dim isolated neutron stars: What do we know?". Max-Planck Institute for Extraterrestrial Physics. Archived from the original on 23rd January 2021.

  9. "The Magnificent Seven: Magnetic fields and surface temperature distributions". arXiv. Archived from the original on 23rd January 2021.

  10. "The Hertzsprung-Russell Diagram". CSIRO. Archived from the original on 23rd January 2021.

  11. "Physicists May Have Found Dark Matter: X-rays Surrounding "Magnificent 7" May Be Traces of Theorised Particle". SciTech Daily. Archived from the original on 23rd January 2021.

  12. "Transparency of the atmosphere". ESO. Archived from the original on 23rd January 2021.

  13. "XMM-Newton Summary". ESA. Archived from the original on 23rd January 2021.

  14. "About Chandra". Chandra X-ray Observatory, Harvard. Archived from the original on 23rd January 2021.

  15. "CAST". CERN. Archived from the original on 23rd January 2021.

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