EM Spectrum in Astronomy

EM Spectrum in Astronomy from Astrobites

I created a mindmap from the information above

  • Radio
    λ > 1 mm
    ν < 300 GHz

    • Objects
      • AGN JEts
      • Supernovae
      • Tidal Disruption Events
      • H II regions
      • Gamma ray bursts
      • Radio Galaxies
    • Processes
      • Synchrotron radiation
      • Free-free radiation
    • Observation
      • Ground based
    • Telescopes
      • Green Bank Telescope (GBT)
      • Five-hundred-meter Aperture Spherical Telescope (FAST)
      • Very Large Array (VLA)
      • Square Kilometer Array (SKA)
      • Low-Frequency Array (LoFAR),
      • Giant Meterwave Radio Telescope (GMRT)
  • Microwave/Sub-mm
    λ ~ 300 μm to 1 mm
    ν ~ 1 THz to 300 GHz

    • Objects
      • CMB
      • High energy phenomena
        • Relativistic jets
        • Cold dust
        • Cold gas
        • Galaxies at high z
    • Processes
      • Thermal (blackbody radiation)
    • Observation
      • Space
      • Ground
    • Telescopes
      • Space
        • Cosmic Background Explorer (COBE)
        • Wilkinson Microwave Anisotropy Probe (WMAP)
        • Planck
      • Ground
        • Submillimeter Array (SMA
        • Atacama Large Millimeter/submillimeter Array (ALMA).
  • Infrared
    λ ~ 300 μm to 2.5 μm
    ν ~ 1 THz to 120 THz

    • Far-Infrared
      λ ~ 15 μm to 300 μm
      ν ~ 20 THz 1 THz

      • Objects
        • Cool dust
        • Cool gas
        • star forming galaxies
        • young stellar objects
          • proto-stars
          • pre-main sequence stars
      • Processes
        • Thermal (Blackbody radiation)
      • Observation
        • Space
      • Telescopes
        • Infrared Astronomical Satellite (IRAS)
        • Infrared Space Observatory (ISO)
        • Herschel
    • Mid-Infrared
      λ ~ 2.5 μm to 15 μm
      ν ~ 120 THz 20 THz

      • Objects
        • Cosmic dust
          • surrounding young stars
          • protoplanetary disks
          • zodiacal dust
        • Solar system objects
          • planets
          • comets
          • asteroids
      • Processes
        • Thermal (Blackbody Radiation)
      • Observation
        • Space
        • Ground
      • Telescopes
        • Ground
          • Infrared Telescope Facility (IRTF)
          • United Kingdom Infrared Telescope (UKIRT)
        • Space
          • James Webb Space Telescope
          • Wide-field Infrared Survey Explorer (WISE)
          • Spitzer
    • Near-Infrared
      λ ~ 0.8 μm to 2.5 μm
      ν ~ 380 THz 120 THz

      • Objects
        • M-dwarfs
        • Cool stars
        • Low-mass stars
        • Galaxies
      • Processes
        • Thermal (Blackbody Radiation)
      • Observation
        • Space
        • Ground
      • Telescopes
        • Ground
          • 2MASS survey
          • Infrared Telescope Facility (IRTF)
          • United Kingdom Infrared Telescope (UKIRT)
          • Visible and Infrared Survey Telescope for Astronomy (VISTA)
        • Space
          • James Webb Space Telescope
  • Optical
    λ ~ 350 nm to 800 nm
    ν ~ 860 THz 380 THz

    • Objects
      • Ionized gases
      • Stars
      • Galaxies
    • Processes
      • Black Body Radiation (Thermal)
      • Non-thermal
    • Observation
      • Both Ground and Space
    • Telescopes
      • Ground
        • W.M. Keck telescopes
        • Very Large Telescopes
        • Southern African Large Telescope (SALT)
      • Space
        • Hubble Space Telescope
        • Gaia
        • Kepler
        • Transiting Exoplanet Survey Satellite (TESS).
  • Ultra-violet
    λ ~ 10 nm to 350 nm
    ν ~ 3e16 Hz to 860 Hz
    E ~ 120 eV to 3.5 eV

    • Objects
      • Thermal
        • O Stars
        • B Stars
        • white dwarfs
      • Non-thermal
        • AGN (continuous emission)
    • Processes
      • Blackbody radiation (thermal radiation)
      • Non-thermal sources
    • Observation
      • Ground (longest)
      • Space
    • Telescopes
      • AstroSAT (2015)
      • Galaxy Evolution Explorer (GALEX) (2003)
      • Hubble Space Telescope (1990)
      • Neil Gehrels Swift Observatory (2004)
  • X-Ray
    λ ~ 10 pm to 10 nm
    ν ~ 3e19 Hz to 3e16 Hz
    E ~ 120 keV to 0.12 keV

    • Objects
      • X-Ray binaries
      • AGN
      • Neutron stars
    • Processes
      • Thermal Emission
      • Free-Free emission
      • Accretion
    • Observation
      • From Space
    • Telescopes
      • Uhuru (1973)
      • Einstein (1978-81)
      • ROSAT (1999)
      • Chandra (1999)
      • XMM-Newton (1999)
      • NuSTAR (2012)
      • eROSITA (2019)
    • Gamma-Ray
      λ < 10 pm
      ν > 3e19 Hz
      E > 120 keV

      • Objects
        • AGN with Relativistic Jets
        • Gamma Ray Binaries
        • Gamma Ray Bursts
      • Processes
        • Gamma Decay
        • Pair-Annihilation
        • Shock Waves
        • Inverse-Compton Scattering
      • Observation
        • From Space
      • Telescopes
        • Compton Gamma-ray Observatory (1991)
        • International Gamma-Ray Astrophysics Laboratory (INTEGRAL) (2002)
        • Fermi (2008)

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Mysterious spiral shells

I have visited one of the most iconic sea forts in Maharashtra – the Sindhudurga at Malvan on the Konkan Coast twice. It is one of the most beautiful sea forts you will witness. The crystal clear waters and blue skies will be imprinted in your memories. The first time I had an analog camera with me, so I couldn’t take as many photos as I would have wanted (one of the analogue snaps is above c. 2001). The second time I had a really nice digital camera with me, so I did take a load of photos (all the ones below).

Maratha navy was a very formidable force on the Konkan coast. The maratha navy under the Angre’s was a force feared by the Portuguese and English alike. They ruled the waters from North of Goa to Colaba Fort (not the same as Colaba in South Bombay) with their capital at Gheria (Vijaydurg), in the later half of 1700s. Maratha navy ships were fastest and very agile in open waters and were absolute terror for the europeans. Anticipating the need for a strong navy as well, Sindhudurga was one of the first sea forts to be built by Shivaji. The fort is built on a rocky outcrop off the coast of Malvan. The fort played an important role after Shivaji as well. It housed Tarabai, the widower queen of younger son Raja Ram during the invasion Mughals. The fort has walls which have stood the test of time and are still standing well even after several centuries against sea water and constant barrage of waves. (though in the second visit we found parts of wall had collapsed). The architecture of the fort is amazing, though we only see the bastions and ramparts now. The fort walls are shaped smoothly when required (in the mathematical sense of continuity), unlike later european forts which are more angular in nature. The front gate is hidden inside a curved pathway between two bastions so that is not visible directly (hence cannot be hit directly with a canon. This mechanism is also seen in several other forts such as Raigad and Janjira. But so much about the fort, coming back to the topic of the post.

On the southern end of the fort there is a small door which opens to a patch of beach. This is one of the best spots on the fort if you are a nature lover. In this small patch there is clear water and beach.

From a bit far
A bit closer to the small door, Sindhudurga Sea Fort Malvan
A bit more close to the small door, Sindhudurga Sea Fort Malvan
Crystal clear waters at Sindhudurga Sea Fort Malvan

The beach sand is coarse, meaning most of it is actually made of seashells. This is a treat to see, myriad shapes, sizes and colours of seashells blended to form the sand. Sand is a fascinating mixture which results from erosion (air, water) and evolution over long time scales. Sand has both inorganic and organic content.

Waves, caustics and crystal clear water…


The sea shells are logarithmic in nature, with the nautilus perhaps being seen as exemplary. In many cases of shells the logarithmic spiral is obvious (see the photo), but in other cases as well it shows logarithmic growth.

Close up of the sand shell mix. In the center, Clypidina notata Linne, Size 23-30 mm.
Close up of the sand shell mix. What is this see weed by the way? Upper left white shell is most probably Cardita antiquata Linn  or Cardium sp.
A multishade rock in the sand shell mix
Close up of the sand shell mix, too
Our spiral shell standing out in the sand shell mix
Close up of the sand shell mix. A hermit crab residing in a Trochus radiatus Gmelin (Banded Torchus)
A view of the ramparts and bastion from the small beach
Close up of the sand shell mix, .
Close up of the sand shell mix
Close up of the sand shell mix. This lot needs a lot of identification!
Close up of the sand shell mix
Close up of the sand shell mix. Sundial (Architectonica laevigata Lamarck) with most probably Cardium asiaticum Bruguiere.
A quadrumvirate of hermit crabs in Dwarf turban (Turbo brunneus Röding) with copper legs in a tidal pool near Aguada Fort, Goa)
A company of pebbles (Anjuna beach, Goa)
A company of pebbles (Anjuna beach, Goa)

See a previous post on patterns in nature, this post is a sort of extension of that. For an extensive and excellent treatise on spirals see The Curves of Life by Theodore Cook. On a side note, you should also read Junijo Ito’s spiral themed horror manga Uzumaki.

I did collect a few noteworthy shells from this patch (there were so many to collect!), and these included a few approximately hemispherical ones which were flat on the other side. These shells had a nice logarithmic spiral on the flat side which was also relatively smooth. While the hemispherical side was comparatively rougher.

Now in all other shells, one could easily visualise where would the animal be that created the shell or how the animals used that shell. for example

Connus mutabilis (Reeve) showing excellent spiral structure, from Madh beach
Cantharus spiralis (Gray), from Madh beach
Turritella duplicata (Lamarck) along with a hermit crab, 
Natica didmya (Röding), Revdanda Beach
Burasa tuberculata (Brodip) Tuberculated Frog, Madh Beach
Surcula javana (Linne) Javan Turrid, Nagaon Beach
Mix of organic and mineral sand, Madh Beach
Mix of organic and mineral sand, Madh Beach
Variants of Umbonium vestiarium (Linne), Button shells
Variants of Umbonium vestiarium (Linne), Button shells, Nagaon beach
Variants of Umbonium vestiarium (Linne), Button shells (from Nagaon beach)
Our mysterious spiral shells
Our mysterious spiral shells.
Our mysterious spiral shells
Our mysterious spiral shells


But in case of these spirals I could not understand how or where the animal would be using this shell. There was no hole to hide or provide a protection to the animal, as it was solid. It was indeed a puzzle. The shells definitely belonged to an animal, but which one and how did it use it? I asked around but did not get any clear answers. Thus began a journey to unravel the mystery. The image search for spiral shell did not help, though I came to know that these shells are used in astrology oriented rings with silver. And apparently they are seen in jewellery shops.

The first thing we knew for sure was that the shell belonged to a marine animal. But how did that animal use this? It remained an unsolved question for a couple of years. I would ask around to anyone with some knowledge of zoology, but it didn’t get any answers. Then one day someone did faintly recognised, “Isn’t this an operculum?” Now a quick image search with this new term, operculum, I learned gave the answer to this mystery. And commonly it is also known as cat’s eye shell. And the long mystery was solved:

The operculum is attached to the upper surface of the foot and in its most complete state, it serves as a sort of “trapdoor” to close the aperture of the shell when the soft parts of the animal are retracted. The shape of the operculum varies greatly from one family of gastropods to another. It is fairly often circular, or more or less oval in shape. In species where the operculum fits snugly, its outline corresponds exactly to the shape of the aperture of the shell and it serves to seal the entrance of the shell. (wiki)

TODO: Identify all the shells in the photos..



Deepak Apte – The Book of Indian Shells (BNHS)



Free Graphics Illustration Resources and Repositories

Finding a good and accurate graphic or illustration for your need is something that we all struggle with. On top of that if the requirement is that the graphic resource has to be free (as in freedom) then it further narrows down the options. Sometimes you see the graphic you want, but its license terms are unknown or are not agreeable to your work as they are not released freely. So what do you do? Either you use an illustration which is not perfect fit or you use one which breaks your resource (in terms of license).  But the problems is also that there are many free resource repositories which are very well not known. I have personally come across great many graphic resources, only to be forgotten after their current need is finished. This post is an attempt to overcome this. This post is a collection of various free graphics and illustration resources and repositories that I have found useful over the years. It is also a sort of personal bookmark list of the these resources if and when I need them in the future. I hope this will be of use to others too. I will keep on updating this list with new resources that I find. If you know of any resources which are missing please do post in the comments.

The Internet Archive Image Search https://archive.org/details/image

Wikimedia Commons https://commons.wikimedia.org/

Open source illustrations kit https://illlustrations.co/

NYPL Public Domain Archive https://nypl.getarchive.net

David Rumsey Map Collection  https://www.davidrumsey.com/  also https://archive.org/details/david-rumsey-map-collection

Metropolitan Museum of Arts Public Domain Images also https://archive.org/details/metropolitanmuseumofart-gallery

Cleveland Museum of Art Open Access also https://archive.org/details/clevelandart

Brooklyn Museum also https://archive.org/details/brooklynmuseum


Unsplash Free Images

NASA Images

ESO Images

Vintage Ads

Vintage Australian Print Ads

Vintage British Print Ads https://archive.org/details/vintage-british-magazine-ads

Vintage American Print Ads https://archive.org/details/vintage-american-print-ads


Vintage Danish Print Ads https://archive.org/details/vintage-danish-print-ads





A library of microorganisms https://archive.org/details/cmpuj


National Gallery of Art also https://archive.org/details/national-gallery-of-art-images

Flickr Collections (there are several collections on Flickr which are openly licensed)

Flickr Commons

NOAA Photo Library https://flickr.com/photos/noaaphotolib/