plant physiology - Compare and contrast "Rubisco activity" and "assimilation rate" (is there a difference, and if so, what is it)?

Both are measures of carbon fixation rate.

RuBisCO activity specifically refers to the rate at which the enzyme RuBisCO fixes carbon to RuBP, and is measured by isolating the enzyme from tissue samples and, usually, using radiolabelled CO2 to measure how much carbon is fixed in a set amount of time in controlled conditions and with controlled supply of substrate.

Carbon assimilation rate is generally a whole-plant or whole-leaf variable, measured on a larger scale. It is usually measured by measuring leaf sugar carbon before and after a set period, again using radiolabelling. Conditions may or may not be controlled, and it can be measured in the field (unlike RuBisCO activity).

RuBisCO activity might differ from net fixation in the short term in plants which have an intermediate carbon fixation step, such as in CAM or C4 plants, if the net fixation measurement includes the intermediates - the pool of intermediate carbon-containing molecules have technically been fixed but not by RuBisCO. However, over longer time periods this effect will disappear, as ultimately all CO2 is fixed or re-fixed through RuBisCO.

space telescope - Could the Earth be used to cast an arago/poisson spot on something

One constraint is the recommendation $F=frac{d^2}{blambda}geq 1$, in this case with $d=12700 mbox{ km}$ about the diameter of Earth, $lambda=600 mbox{ nm}$ some wavelength of visible light, and $b$ the distance between circular obstacle and observer.
The distance between Earth and the observer should hence be

$bleq frac{d^2}{lambda}=frac{(12700cdot10^3mbox{ m})^2}{600cdot 10^{-9}mbox{ m}}=481.67cdot 10^{18}mbox{ m}$

Another constraint is the surface roughness of the circular object:
$Delta r < sqrt{r^2 + lambdafrac{gb}{g+b}}-r$, with $r=6350mbox{ km}$ the radius of the circular obstacle (here Earth), $g$ the distance between the point light source and the circular obstacle, and $b$ the distance between the circular obstacle and the screen.

To simplify calculations, say $ggg b$. Then approximately
$Delta r < sqrt{r^2 + lambdafrac{gb}{g}}-r = sqrt{r^2 + lambda b}-r$.

After adding $r$ and squaring you get
$(Delta r +r)^2<r^2+lambda b$.
This simplifies to
$(Delta r)^2+2rDelta r<lambda b$.
Assume $Delta rll r$, and neglect the second order $(Delta r)^2$ to get
$2rDelta r<lambda b$. Divide by $lambda$ to get an approximate constraint for $b$ as

$b>frac{2rDelta r}{lambda}$.
With $2r=12700 mbox{ km}$ about the diameter of Earth, $lambda=600 mbox{ nm}$ some wavelength of visible light, we get

$b>frac{12700cdot 10^3mbox{ m}cdotDelta r}{600cdot 10^{-9}mbox{ m}}
= 21.1667cdot 10^{12}Delta r$.

The two constraints allow for reasonable values of $Delta r$.
Assume a surface roughness of Earth of e.g. $Delta r = 1mbox{ km}$.
Then a valid range of distances of observers would be between
$0.00224$ and $50912$ lightyears of $9.4607cdot 10^{15}mbox{ m}$ from Earth.

In astronomcal units of $149597870700mbox{ m}$ the closest distance of an observer would be $141.49 mbox{ au}$ from Earth.

Due to Earth's oblateness, however, you would get a point spread function significantly different from a dot for this "short" distance from Earth. It might be possible to correct this by an appropriate telescope optics.

The effect of gravitational lensing is
$theta=frac{4GM}{rc^2}=2.969cdot 10^{-27}frac{mbox{m}}{mbox{kg}}frac{M}{r}$, after applying the constant of gravitation $G$ and the speed of light $c$. With the mass $M=5.97237cdot 10^{24}mbox{ kg}$ and a radius of $r=6350000mbox{ m}$ of Earth, we get an angle of
$theta=2.793cdot 10^{-9}$ by gravitational lensing at the surface of Earth.

This would focus parallel rays of light to a point near a distance of
$b=frac{r}{tan theta}=130.27cdot 10^{15}mbox{ m}$, or $13.77$ lightyears, hence well beyond the minimum distance where an Arago spot could form. But, of course, the innermost peak of the point spread function would be closer to a circular disc at this larger distance with relevant gravitational lensing.

observational astronomy - SDSS Image FITS files have negative values. What do these negative values mean?

Without further details I would guess your fits file is using 16 bit signed integers between -32768 and +32767.

In which case you should find that the fits header item called bzero has been set to 32768.

The true integer values are given by your pixel value*bscale + bzero.

This is a very common format for raw data from CCD cameras.

On the other hand if you are talking about processed sky subtracted SDSS images, then of course some pixels are negative - the average pixel in a "sky" region will be zero, so a substantial fraction must be negative.

human anatomy - Why can hair grow without limit while eyebrow cannot?

Hair actually does have a limit to its growth. All hair cycles between periods where it grows, and when a new stand appears, which pushes the older, long hair out. Hairs that are generally shorter, like eyebrows or arm hairs, have a shorter growth period before a new hair pushes the old one out. The hair on your head actually grows much faster than many other hairs as well, so with the longer cycle and the faster growth, it can grow to be much longer before being replaced.

I remember hearing that for many women (the cycle does vary from individual to individual, which explains why some men can't grow beards while others look like ZZ Top) the growth cycle ends between the waist and butt.

My speculation would be that hair on the head could grow for longer because there are more capillaries in the scalp, which might lead to more nutrients, higher cell turnover (like you see in dandruff), and hence more hair. That's just a guess, mind you.

zoology - Is there any reason the common housefly continues to return to an area?

I don't think it's a silly question, but it is a common error to anthropomorphise animals.

Insects respond to cues which they have evolved to respond to, and this is how they 'make decisions'. They do not have free will or any more complex decision-making process like common sense. This is evident is lots of insect behaviour: flying repeatedly at a closed window; landing on brightly coloured clothes instead of flowers; and returning to a food source when they are in real danger of being swatted!

When a fly senses the food (often by olfactory receptors), they are 'programmed' to fly towards it in response to some chemical they sense depending on the species and food. They may not have adapted a response to swatting, or perhaps the food cue overrides others. In nature swatting is not so much of a threat to a fly. Some animals may brush them away, but since they are not really doing any harm in feeding from another animal's food, they are mostly ignored.

CO2 traps are used to entice and kill mosquitoes. The mosquitoes are attracted to CO2 (as it is dispelled from the animals from which they blood feed), they will only evolve to avoid the traps if there is another cue which they could eventually associate with a negative effect.

Another thing to remember when thinking about insect behaviour is that their life strategy is very different to ours. Insects are more r-selected than humans, meaning that each individual life has not had so much energy put into it as a more K-selected animal (such as humans), and to compensate for this, many more young are produced. This often results in more risks being taken by individuals since there will still be a viable population even after many deaths.

Chapter 4 of 'The Insects' by Gullan & Cranston gives a good introduction to the sensory responses of insect behaviour. There are other books on the subject, 'Introduction to insect behaviour' by Atkins looks like a good starting point, but I have not read it yet.

distances - How do we know galaxy GN-z11 is as far away as it is?

So GN z-11 is the latest "furthest away" galaxy. It has a claimed redshift measurement of $z=11.1$, meaning that we are seeing the light it emitted about 400 Myr after the big bang (dependent on an assumed set of cosmological parameters).

To get the redshift measurement, the discoverers used grism (relatively low resolution) spectroscopy in the near infrared. What they were looking for is the rest-frame Lyman alpha continuum break, which would be redshifted into this wavelength range.

The Lyman alpha continuum break is caused by the absorption of nearly all ultraviolet photons with wavelengths smaller than 121 nm. These high energy photons are capable of photoionising the neutral hydrogen present in the intergalactic medium at a range of lower redshifts. This neutral hydrogen is present in abundance at redshifts greater than 6, as it had not yet been re-ionised by quasars and starlight. The consequence is that no light is expected to reach us from rest-frame wavelengths shortwards of 121 nm, but a galaxy's light can reach us from rest-frame wavelengths that are longer than this. When one observes the spectrum, we see flux at long wavelengths which suddenly cuts off at shorter wavelengths. The wavelength of the break is $lambda = 121 times (1+z)$ nm, where $z$ is the redshift.

It is this Lyman alpha continuum break that has been identified at an observed wavelength of 1470 nm. This leads to the redshift estimate of $z= (1470/121)-1 = 11.1$.

The details are presented in Oesch et al. (2016). The continuum break is the only thing visible in the spectrum. The authors are confident that this is what it is because their previous broadband photometry had given them an estimated redshift of $>10$ (which is why they observed this candidate in the first place).

homework - Difference between biological control and introducing species for conservation?

Biological control does not have to be with an introduced species. It can also be accomplished by either artificially inflating the number of existing predators.

E.g. Spruce bud worm has a natural predator in the form of a tiny wasp. But budworm can spread through a stand faster than the wasp can. By moving popluatins of the wasp to the forefront of the budworm advance, you get most of the 2nd generation budworms. The trees have a bad needle year but aren't killed.

It can also be done with habitat modification. If you create conditions that favour the existing predator, then there are more predators in place. E.g. create places for ladybug overwintering to reduce aphid populations. (I have no idea if this would work.)

Anohter example -- although ecological controll may be better than biological -- I had a significant mosquito problem. Lots of small puddles in spring. I created a permanent pond. This has resulted in much smaller mosquito population.

• The pond is a local year round water supply for insectivorous birds.


• The pond is by far the largest water body around, so tends to get most of the mosquito eggs.


• The pond now has a permanent population of boreal and chorus frogs. Tadpoles of these make it tough to be a mosquito larvae.

Is this species introduction? Only if you consider immigrant species from neighborin populations to be introduction.