# Stars/Galaxies/Laboratory

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This is a composite scanned spectral image of NGC 6946. Credit: NASA / CXC / MSSL / R.Soria et al, Optical: AURA / Gemini OBs.

This laboratory is an activity for you to create or define a galaxy. While it is part of the astronomy course principles of radiation astronomy, it is also independent.

Some suggested galaxy entities to consider are the early Hubble classification, electromagnetic radiation, neutrinos, mass, time, Euclidean space, Non-Euclidean space, and spacetime.

More importantly, there are your galaxy delineating entities.

You may choose to define your galaxy entities or use those already available.

Usually, research follows someone else's ideas of how to do something. But, in this laboratory you can create these too.

## Evaluation

evaluation activity

Okay, this is an astronomy, galaxies, laboratory, but you may decide what an astronomy, a galaxy, or a laboratory is, its appropriateness, and applicability.

Yes, this laboratory is structured.

I will provide an example of a possible galaxy and analyze it. The rest is up to you.

Questions, if any, are best placed on the discussion page.

## Control groups

For creating or discovering a galaxy, what would make an acceptable control group? Think about a control group to compare your galaxy or your process of creating a galaxy to.

## Galaxy verification

According to NASA,[1] "NGC 6946 is a medium-sized, face-on spiral galaxy about 22 million light years away from Earth. In the past century, eight supernovas have been observed to explode in the arms of this galaxy. Chandra observations (purple [in the image at the top right of the resource]) have, in fact, revealed three of the oldest supernovas ever detected in X-rays, giving more credence to its nickname of the "Fireworks Galaxy." This composite image also includes optical data from the Gemini Observatory in red, yellow, and cyan."

But is it a galaxy?

### Orientation

The composite image does appear to conform to a face-on galaxy.

### Symmetry

This is an optical/visual image of NGC 6946. Credit: Aladin at SIMBAD.

From question 1 of the galaxies/Quiz and the image at the top of this laboratory, NGC 6946 does not appear to fall easily into any of the six forms of rotational symmetry having about 5 spiral arms on the left and maybe 2 on the right.

Further, the image in this section from SIMBAD appears to have two-fold rotational symmetry with three spiral arms on each side.

### Morphology

The primary source used by SIMBAD considers the nebula to be one of 30 nearby spiral galaxies.[2]

"Targets span a wide range in Hubble type, star formation activity, morphology, and inclination."[2]

### Astronomic distances

Distance moduli have been estimated for NGC 6946 using its brightest blue stars and its HII ring.[3] Its distance modulus is estimated to be log D0 = 4.434.[3] The distance in parsecs is given by

${\displaystyle d=10^{{\frac {\mu _{0}}{5}}+1},}$

where µ0 = 29.25.[3] NGC 6946 is at 106.85 (7.08 x 106) parsecs, or approximately 23.1 x 106 light years. While this is greater than the NASA number, it is not an order of magnitude greater or smaller.

### Classification

In the galaxies lecture is a composite image of Hubble's classification scheme for galaxies. Using this as a visual guide and examining the multispectral image at the top of this resource, NGC 6946 appears to be close to type Sb (example, NGC 2841).

Looking up "NGC 6946" on SIMBAD, without the quotes, reveals that SIMBAD considers NGC 6946 to be an "HII Galaxy" of morphological type "SAB(rs)cd".

A more extensive classification scheme starting from the Hubble scheme indicates that an Sab galaxy is approximately in between Sa and Sb.[4]

### Special characteristics

H II ring(s) and/or regions have been noted.[3]

## Report

Title: Evaluation of nebula NGC 6946.

Author: --Marshallsumter (discusscontribs) 00:19, 9 February 2014 (UTC)

Abstract:

Starting with a nebula classification scheme developed by Edwin Hubble in 1926, a morphological assessment of the nebula NGC 6946 has been made. From observational structure recorded on two independent images, NGC 6946 appears to be a face-on spiral galaxy of type Sab at approximately 23 million light years.

Introduction:

In many nebula images and photographs it is often difficult to determine whether stars or relatively diffuse gaseous or molecular clouds are being imaged. Using additional observations and deductions, even if qualitative, previous claims regarding nebula NGC 6946 are investigated.

Experimentation:

Although nebula NGC 6946 is not known to rotate during observation in the plane of view or vertical to it, its appearance in both images is assessed using the Hubble scheme and forms of rotational symmetry described in question number 1 of galaxies/Quiz. Orientation is determined from image structure and symmetry.

Additional individuals have recorded their opinions on morphology and an assessment based on the Hubble scheme has been estimated.

A distance calculation from 1978 based on secondary indicators has been made.

Several primary sources are consulted regarding classification and special characteristics.

Discussion:

Earlier orientational analyses confirm that the nebula is a face-on galaxy.

Close-up symmetry analysis suggests that a composite image of the nebula from several spectral ranges has no rotational symmetry yet appears spiral-like in morphology.

Distance calculations and gross classifications appear to be supported by independent primary sources.

Special characteristics of H II ring(s) or regions are confirmed by at least one primary source.

Conclusions:

Although NGC 6946 has not apparently moved in its orientation over recent human observations, the nebula appears to be a face-on spiral galaxy of type Sab or Sb at approximately 23 million light years.

## Evaluation

To assess your galaxy, including your justification, analysis and discussion, I will provide such an assessment of my example for comparison.

Evaluation

The finer notations of galaxy classification "(rs)cd" have not been examined or explained. Other independent radiation astronomies have not been consulted for images of the nebula. While several primary authors report stars at great distances indicating a galaxy rather than a spiral star cluster within the Galaxy, individual representations to show this to be the case have not been directly presented or evaluated.

## Hypotheses

1. Just because it looks like a galaxy does not mean it is one.