Astrophysics - "How to swallow a Sun" in Scientific American of April 2017

This document contains comments about the article How to swallow a Sun by S. Bradly Cenko and Neil Gehrels In Scientific American of April 2017.
If the recent discovery of gravitational waves emanating from the early universe holds up under scrutiny, it will illuminate a connection between gravity and quantum mechanics.


Yet they are always hungry and whenever something sizable does happen to fall in, the resulting feeding frenzies can be seen from far away.
To call BH's hungry is to popular. The whole issue is to what extend such a collision can be observed.
In 1988 astronomer Martin Rees proposed a third way to study study super massive BH's etc. He propesed bright flares of light from a BH's vicinity. Called tidal disruption events (TDE's) these outburst occur when a super massive BH consumes an unlucky star.
The fact that this happens is nothing special because that is the only way that a small BH becomes a large BH.

How to destroy a star

When a star is in the vicinity of a supermassive BH - perhaps pushed there by the gravitational nudging of another nearby star - the intense tidal forces can rip it to shreds.
I doubt if this is the correct picture. To break a star into pieces you need different forces on opposite sides of the star.
The very lagest supermasive BH's, those containing billions of solar masses, are too big to easily create TDE's - they swallow stars whole before the tidal forces becomes large enough to tear the stars apart.
The Comet Shoemaker-Levy 9 which collide with Jupiter broke appart in 9 pieces before it collided with Jupiter.
The tidal forces around a black hole with millions of solar masses, will tear apart most stars that approach within about 50 million km etc.
IMO this whole issue depents very much about the mass of the infalling object.
Basic orbital mechanics dictates that roughly half of the debris will be expelled as long filaments of material streaming from the vicinity of the black hole, wheareas the other half will loop back around to form an accreation disk,
The painted piscture is that half of the mass will not merge with the BH and the other half will. IMO in due time 100% will merge with the BH.

Death By Black hole

Page 32 and page 33
On the top of this picture we read:
Stars wandering too close to these cosmic monsters can be shredded by intense gravitational fields, sending gas streaming into the black hole
This sentence should read: "Stars which come too close" etc.
The whole process is very similar as a comet which comes too close to our "Sun"
these outburts are seen all across the cosmos and provide insights into how suppermassive black holes feed and grow.
Also this is nothing special. This can happen with all types of stars.

The first discoveries

Thosesame models also suggested that astronomers could use a TDE to make relatively precise estimates or the mass of its accompanying BH - a critical data point for learning how a BH's size alters its behavior and effects on its galactic surrounding.
The first question to answer is to what extend a TDE is a standard candle. I doubt this. The behavior of a star or any object when a stars breaks apart can be very different in each case.

New questions for a new era

At right side of page 34, almost at the end, we read:
As astronomers sorted out through through these muddled possibilities etc one thing became perfectly clear: TDE's were much more complicated phenomenon than anyone had previously appreciated.
I'm not amased. They are not standard candles.
In theory there are two extreme different possibilities: In both cases the observed phenomena can be quite different.

A shock from shift

The death of worlds

They may even eventuallyu learn about a star's companions, too - planets swallowed by black holes.
I think that it is too optimistic that you can learn a lot when a planet collides with a black hole.
Each flickering flash from a faraway galactic center could signal the deaths of entire worlds.
This is a too populistic claim.
Even if they were not directly consumed, planets could still find themsleves in the path of the transient relativistic jets produced by some TDE's
When an approaching star is accompanied by three or four planets, than these planets will be torn apart and each planet will follow its own trajectory towards the BH and collide somewhere. I do not think that based on the debris observed (if?) it will be very difficult to claim something (composition) of each individual galaxy.

Reflection 1 - Binary systems

Generally speaking there are two types of binary systems:
  1. Where there is a large difference in mass. This is the same situation as with the stars in our Milky Way, which circulate around the BH in the center.
  2. Where the two objects of almost of the same size. This is the case with binary BH systems.
In the article the first situation is mentioned where the infalling star and the smallest star of the binary system are almost of the same size.

Reflection 2 - Collisions

In the article there is made a difference in behavior for BH's of one million versus one billion solar masses.
IMO both BH's will grow continuously in mass caused by infalling objects. Most of these objects will not break apart in the sense discribed in the article.

Reflection 3 - What do we learn.

The article "How to swallow a Sun" studies the situation when a Sun sized object collides with a single BH. This is interesting, but IMO it is much more interesting to study the situation where two BH's are involved, which can lead to the merging of two BH's
This situation is described in the document Visual Basic program "VB BHmerger" - Description and operation

If you want to give a comment you can use the following form Comment form
Created: 1 August 2015

Back to calling page Comments About Scientific American
Back to my home page Contents of This Document