Supernovae - novae - average duration ?

When supernovae or novae are reported that are visible to amateur class
scopes, is there a rule of thumb for how many days they will remain
visible? What is the average duration of the visible light-curve?

Recent examples include sn2005cs in M51, which can be still be imaged
by astrophotography although its discovery date was June 26. Nova Sgr
05 #2 was discovered on July 5 and I understand is still visible.

- Canopus56

A "typical" supernova will remain within 3 magnitudes of
its peak light for one to three months. One might expect
a typical supernova to remain bright for several months
after discovery; however, if the event was discovered
long after its maximum, it might fade to invisibility
just a few weeks after it is found.

Classical novae tend to evolve more quickly than
supernovae: they might drop 3 magnitudes from
peak in just a week or two. However, since some
novae in our own Milky Way reach brighter
apparent magnitudes than most supernovae --
the Nova Aql 1999 number 2, for example, reached
mag 4 -- they might still be detectable in small
telescopes despite dropping 5 or 6 magnitudes
from their peak.

Michael Richmond

X-No-archive: yes

Thanks, Michael, for your well-written answer. - Canopus56

Stupendous_Man wrote:
A "typical" supernova will remain within 3 magnitudes of
its peak light for one to three months. . . .
Classical novae tend to evolve more quickly than
supernovae: they might drop 3 magnitudes from
peak in just a week or two.

Stupendous_Man wrote:
A "typical" supernova will remain within 3 magnitudes of
its peak light for one to three months. One might expect
a typical supernova to remain bright for several months
after discovery; . . .

Classical novae tend to evolve more quickly than
supernovae: they might drop 3 magnitudes from
peak in just a week or two. . . .

Michael, I'm having problems conceptualizing why it is an SN would
remain 3 magnitudes below it peak explosion brightness.

For a hypothetical, wouldn't a mag 17 pre-explosion supernova just
explode - there's a flash to mag 13 - and that's it. Then the star
falls back to its pre-explosion magnitude.

Or is there a superheated cloud of expanding stellar remnants that
keeps the magnitude at a higher base-line than pre-explosion? The
magnitude decreases proportionally as the size of the cloud increases.

Ditto for classical noave.

What is the process that holds the SN's and N's magnitude at a higher
baseline?

- Canopus56

Kurt (canopus56) wrote:
Michael, I'm having problems conceptualizing why it is an SN would
remain 3 magnitudes below it peak explosion brightness.

For a hypothetical, wouldn't a mag 17 pre-explosion supernova just
explode - there's a flash to mag 13 - and that's it. Then the star
falls back to its pre-explosion magnitude.

Or is there a superheated cloud of expanding stellar remnants that
keeps the magnitude at a higher base-line than pre-explosion? The
magnitude decreases proportionally as the size of the cloud increases.

That's partly it. What often happens is that nucleosynthesis doesn't
quite finish right at iron-56. If it did, a Type Ia would be a quick
flash and then the remnants would expand so quickly that they would
dim very rapidly.

Instead, what seems to happen is that nuclides close to iron-56 but
not right at it are formed, and these decay relatively slowly over
the next couple of months to supply some of the brightness. One of
those apparently is cobalt-56, which decays to iron-56 with a half
life of a couple of months (I think), and Type Ia supernovae do seem
to have one exponential decay with a half-life of a couple of months.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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Brian Tung wrote:
Instead, what seems to happen is that nuclides close to iron-56 but
not right at it are formed, and these decay relatively slowly over
the next couple of months to supply some of the brightness.

Thanks for your excellent reply. I missed it when you originally
posted and have only read it today. It's still timely, considering that
2005cs is shown by the light-curve at the SNWeb site as holding steady
at about 14.3 mags -
http://www.astrosurf.com/snweb2/2005/05cs/05csCurv.htm
http://www.rochesterastronomy.org/sn2005/sn2005cs.html