Determining the Age and Distance to the Open Cluster NGC 2194

Abdullah Pehlari, Luis Aceves-Ramirez, Yashica Balasubramanian, Rodrigo Burguete, Ashwin Krishnamurthy, and Cindy Wang

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Mentored by Ms. Olivia Kuper

What are Open Clusters?

Credit: Wikipedia

Credit: LearnTheSky

• Groups of a few dozen to a few thousand stars that formed together from the same giant molecular cloud�
• Loosely bound by gravity, so unlike globular clusters, they’re more spread out and can eventually drift apart�
• Typically found in the disk of a galaxy (like the Milky Way), in the spiral arms where star formation happens

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• Share similar ages and chemical compositions because they came from the same material�
• Pleiades (Seven Sisters) and the Hyades in Taurus

Yashica

Life Cycle

• Formation & Early Phase

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• Stars form from collapsing gas and dust in a giant molecular cloud, creating a loose group of dozens to thousands of stars
• Massive newborn stars quickly clear leftover gas with strong winds or supernovae, shaping the young cluster

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• Stabilization

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• Once leftover gas is pushed out, cluster settles into a more stable gravitational arrangement, still loosely held together
• Cluster stars share age and composition; over time, they evolve off the main sequence based on mass�
• Dissolution & Dispersal

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• Over time, gravity from nearby clouds, stars, and the galaxy pulls the cluster apart, scattering the stars into the galactic field

Yashica

Structure & Composition

Consists of young, metal-rich Population I stars formed from the same molecular cloud. Often includes massive blue stars. Younger clusters have higher metallicity.

Loosely bound group of stars spanning a few light-years, with an irregular or spherical shape. Located in the galactic disk. Prone to dispersal due to weak gravitational binding.

Structure

Composition

Yashica

Importance

Credit: Wikipedia

• Useful for comparing how mass affects star evolution�
• Helps calibrate stellar age models

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• Elemental makeup tells us about the interstellar medium at that time/place�
• Studying their distribution helps map the Milky Way’s spiral arms and disk evolution�
• Over time, heavier stars sink to the center. This teaches us about gravitational interactions and internal cluster dynamics

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Yashica

Problem/Goal

Problem: Lack of precise data on the physical properties of stars within distant open clusters around 10,000 light-years away

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Gap: Limits our ability to accurately determine the cluster’s age and distance

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Importance: Open clusters can help improve models of how stars evolve and measure distances in the galaxy

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Goal: To determine the age and distance of an open star cluster located ~10,000 light-years away (NGC 2194)

Credit: ESO

Yashica

Data Collection

Images of NGC 2194 were taken with remote observation on the SARA (Southeastern Association for Research in Astronomy) telescope in Cerro Tololo, Chile.

• 0.6 meter aperture
• Full access of the Southern hemisphere sky
• Reflecting telescope

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Credit: NoirLab

Credit: NoirLab

Credit: NoirLab

Cindy

Telescope Properties

Advantages

• No chromatic aberration
• Easier and cheaper to scale
• Can observe fainter objects compared to refracting telescopes

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Disadvantages and Challenges

• Atmospheric turbulence, disturbance (seeing)
• Internal air currents
• Temperature effects
• Interstellar medium

Credit: Teach Engineering

Credit: Stanford Kavli Institute

Cindy

Astronomical Filters

• Block the color spectrum above and below a certain bandpass
• Increase the signal-to-noise ratio of important wavelengths

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Narrowband filters: study light from certain elements (H/O)

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Broadband filter types: UVBRI

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Credit: Tawalbeh & Al-Wardat

Cindy

Our Filters

Astronomical images of NGC 2194

• Six images
• Three filters: B (blue), V (visual, or green), R (red)
• Two exposure settings: 3 and 120 seconds

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Filters are broadband, allowing for analysis such as main sequence fitting.

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For our purposes, only the blue and green filter data were necessary.

Credit: Tom Rutherford

Ashwin

Methodology

Photometry:

Analyzed 120 second exposures of B and V filter images in AstroImageJ:

• Identified reference star(s) and magnitude(s)
• Placed apertures to measure target star magnitudes
• Recorded data in a spreadsheet

Ashwin

Photometry

In AstroImageJ, the software calculates a star’s apparent magnitude by counting the number of photons (pixel values) detected within a defined aperture and converting that into a brightness value, which can then be compared across filters.

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The B (blue) and V (visual) filters , which were used for this cluster, let through only blue and green-yellow light, respectively, allowing us to measure how bright a star appears in those specific wavelengths.

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Once raw images are taken of the open cluster from telescopes with filters, further processing is required to extract meaningful information. With the magnitude values of stars from both filters, we were able to classify the stars into spectral classes and thus their ages.

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AstroImageJ was a careful process that took several trials to get clean results, and although finicky at times, it was a huge help!

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Credit: AAVSO

Our Reference Stars

Rodrigo

Preparation

Our initial step was to develop a consistent numbering system for stars within the open cluster.

• Used The Galactic Clusters NGC 2126 and NGC 2194 (Cuffey 1942) as a reference.

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• Divided cluster into rings and assigned each ring to a team member.

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• Unlike Cuffey’s study, which was limited to a small subset of the brightest stars, our modern data included far more stars, allowing for a complete analysis of all 816 stars in the cluster.

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• This larger sample allowed us to estimate the cluster’s age and distance more accurately than was possible in 1942.

Credit: Cuffey (1942)

Abdullah

Star Classification

We classified stars using their color (B–V) and brightness (V)

• The Inner Ring was a dense central cluster, with a mix of F, G, K, and many M-type stars, which shows a range from yellow to red dwarfs, with temperatures roughly 2,000 K–7,000 K.

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• The Second & Third Rings included mainly A‑type and F‑type stars (~7,500–10,000 K), as well as cooler K-type and G-type stars. We rarely recorded O-type and B-type stars.

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• The Fourth and Fifth Rings contained less stars, and consisted of largely colder K-type and M-type stars.

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Abdullah

Research Findings

Our H-R diagram revealed the age of our cluster

• Most stars lie along the main sequence, confirming they are in the hydrogen-fusing phase.

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• A clear turnoff point near spectral type B–A indicates where higher-mass stars are evolving off the main sequence.

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• Comparing the turnoff with stellar models, we estimate the cluster’s age to be ~30 million years.

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• The mix of F, G, K, and many M-type stars shows a varied stellar makeup, with cooler, lower-mass stars concentrated in the inner regions.

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Abdullah

Reddening

What is reddening?

• When light from stars passes through interstellar dust and gets altered.
• The blue light coming from distant objects is strongly absorbed and scattered by the dust, essentially removing it from the light reaching us and making the objects appear redder than they really are.

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How we measured for reddening:

• We can determine the degree of reddening by analyzing the color index (B-V) of an object and comparing it to its true color index (B-V)

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Why it matters?

• It help use accurately determined the cluster’s age and distance.

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Luis

Credit: Chris Mihos

Finding Age and Distance

We estimated the turnoff to be roughly (0,13.5).

• Our turn off color index of 0 is closer to B Class stars, meaning the The approximate main sequence lifetime and the age of NGC 2194 is 30 million.
• Then using the distance modulus equation,to determine the distance of the cluster we need to use the distance modulus equation.
• The d is the distance in parsecs, m is the magnitude, M is the absolute magnitude, which is unknown, so we need a star that is known as a reference, which will be our sun with (B-V) value of .65, V magnitude of 4.83.
• And using a similar star in our cluster with a corrected V value of 14.9894.
• Then we substituting the variables into the equation, we get 10^((14.9894-4.83+5)/5), and we get the distance of our cluster approximately 1,076.17 parsecs.

Credit: University of Washington

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Luis

d = 10^(m-M+5)/5

d = distance in parsecs

m = the star’s apparent magnitude

M = the star’s absolute magnitude (V corrected)

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Plugging the parameters into this equation, we get d = 10^{(14.9894-4.83+5)/5} = 1076.17 parsecs = 3509.99 light years away

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Comparison

Since in 1942 there is limited technology compare to modern technology access, we found that the difference are in…

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• Number of clusters stars, we numbered 816 stars as part of the cluster while Cuffey numbered 120.
• Distance of the cluster, Cuffey determined they were 1200 parsecs, while we determined it was around 1100 parsecs
• Cuffey and our main sequence turnoff got similar spectral type star, a B class.

What we expect differently in Cuffey (1942) photometry to ours:

Photometric accuracy, stars detected, estimate distance and reddening.

Luis

Credit: Cuffey (1942)

Reflections

Importance of our study

• Refine stellar evolution models
• Aids in luminosity calibration
• Physical dynamics in gravitationally bound systems
• Lays a pipeline for examining other open clusters

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Highlights

• Learning astronomical software AstroImageJ and Stellarium
• Collaborating with driven students
• Conducting authentic and valuable space research

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Cindy

Credit: AstroBin

Acknowledgements

Thank you so much for the invaluable help and support!

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• Dr. Olivia Kuper for her mentorship and advice
• Mr. Thomas Rutherford for data collection
• Southeastern Association for Research in Astronomy
• University of Texas at Austin Center for Space Research
• National Aeronautics and Space Administration
• NASA STEM Enhancement in Earth Science Internship Program

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