# Estimating the effective grammage of the galaxy

• pastro
In summary, the conversation discusses the effective grammage of the galaxy along the line of sight. The speaker claims a value of 10^-3 g/cm^2 based on their calculation, but the text estimates a value 100 times lower. The speaker questions the proper model of galactic density and suggests that the author may have used the thickness of the galactic disk instead of the radius as a reference. However, this creates a discrepancy between the speaker's calculation and the context where the number was read.
pastro
I was reading a text which gives (without derivation/reference) the effective grammage along the line of sight through the disk of the galaxy as about 10^-3 g/cm^2. Is this the proper order of magnitude for the accepted value?

By my calculation, I claim the galaxy has 1 proton/cm^3 ~ 2 x 10^-24 g/cm^3 , and a radius of 15 kpc ~ 5 x 10^22 cm. I get a grammage (=density x galactic radius) of only 10^-1 g/cm^2--100 times greater than the estimate of the text. Of course, I'm assuming the line of sight only passes through ISM.

I suppose my crude estimate just isn't good enough. Technically, grammage will be the path integral of galactic density along the line of sight, but what is the proper model of galactic density? Could someone provide insight on the cause of this inconsistency? I would have though my estimate should be closer than it is.

Thanks!

I suppose the author might have used the thickness of the galactic disk instead of the radius...that gets things about right...but the context where I read this number needs an upper limit, and using the thickness of the galactic disk gives a lower limit.

## 1. How is the effective grammage of a galaxy estimated?

The effective grammage of a galaxy is estimated by measuring the distribution of elements throughout the galaxy and calculating the average mass of those elements. This is typically done through spectroscopic observations of the light emitted by stars and gas in the galaxy.

## 2. What factors affect the effective grammage of a galaxy?

The effective grammage of a galaxy is affected by several factors, including the amount and type of elements present, the density and distribution of stars and gas, and the history of star formation and chemical enrichment within the galaxy.

## 3. How does the effective grammage of a galaxy relate to its size?

The effective grammage of a galaxy is not directly related to its size. While larger galaxies may contain more stars and gas, the distribution of elements and their average mass can vary greatly between galaxies of similar size.

## 4. Can the effective grammage of a galaxy change over time?

Yes, the effective grammage of a galaxy can change over time as new stars form and older stars die, altering the distribution of elements. Additionally, interactions with other galaxies can also affect the effective grammage of a galaxy.

## 5. What is the significance of estimating the effective grammage of a galaxy?

Estimating the effective grammage of a galaxy is important for understanding its evolution and formation, as well as the processes that drive the formation and evolution of galaxies in general. It also provides insights into the chemical composition and potential habitability of the galaxy.

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